How much is essence worth?

These are all very good questions and I for one don't value essence as much as others in the game. I would have probably taken the 2 / 6 forest next to water than the 3 / 3 / 2, but that is me and others have different play styles. I tend to give myself more cities when I have the opportunity as well, since I like the multiple cities (I worry less about the unrest penalties than others). But, if I have no cities with essence I will choose the  essence, as you need at least 1 city with essence.

It depend on what type of city you need in that moment.

If I'm looking for a Conclave, I'll go for the 3/3/2, if I need a Town, I'll go for the 2/6.

For building units, it's really powerful. As Pariden or my Pariden-based custom faction, I can get 4 essences and use them all to build super enchanted units which dominate. 

In the particular situation you describe, it would, not knowing of other factors, be a tough call. However, essence is generally superior to grain and material. It depends on a number of factors, though, such as what spells you have available to cast on a city, what your overall situation is etc. Also, the later in the game you are, the better essence becomes (due to spells such as Arcane Forge).

Essence is generally extremely powerful, probably even too powerful. Beyond the other bonuses a city with essence offers it can almost always make up for the 'lost' grain/material through spellcasting while usually also having room to net you a large chunk of mana. I do believe a rebalance may be required.

Early game? The 2-6. Late game? Essence. Arcane forge will up mats to 5.

Gentle rain for more food. Etc. With sorting pool it gets even better. 

Essence gives you flexibility, while that 2-6 is an awesome pioneer spammer  

I would almost always go for the essence location due to its flexibility.

Like Sjaminei said, Essence cities can be slow starters: less material and grain hurts growth and building. Especially the difference between conclaves and fortresses (with higher material yield) is very noticeable.

If you need a Town, I'd take the 2/6/0.  If you need a Fortress or Conclave, I'd take the 3/3/2.  

Towns just don't really benefit much from Essences.  Sure, you can spend 110 mana to make it seem like a 3/6/0, but chances are good that I don't have 110 mana to spare.

Fortresses need Essences.  If it's going to produce units, you want enchants that improve the units, because they make a big difference.  If you just want the level-up bonuses, the enchants to improve growth are key. 

Conclaves rely heavily on Essences due to the Herbalist upgrades (the Herbalist itself doesn't, just the upgrades).  

It should also be noted that some buildings require the city to have at least one Essence to be built, such as the Cleric.

Arcane Forge and Lvl 5 Town Upgrade (5 Gildar pr material) While coming in late, this can net you a huge amount of gildar income in the right spot. (and of course when AI's do stupid things like making a big town in high essence places ) Prioritize Fortresses and Conclaves first, but it's not necessarily foolish to make a town with high essence.  

I would revise this to say that "Troop-training Fortresses need Essence." Not all of the fortresses I get are needed for troop-production - some I get for map control, some for unrest reduction, and some for building armies, but not all of those necessarily go hand-in-hand. If I already have one or two Fortresses with decent amounts of Essence, I don't really need another one, so after I have my one or two high-essence fortresses I'm not going to put down more fortresses on high-essence sites unless I decide I need more troop production.

I'd tend to agree that towns don't benefit too much from essence, but it depends on what you put into the town. Propaganda and Arcane Forge on a town specifically built for wealth production are great, and extra food on a town built for rapid growth is useful (but only for feeding the sacrifice spells), and any spell that boosts production is always useful on any city. I'd still rather use essence sites for conclaves and training fortresses, but you only really need so many of those, and once you get to that point you might as well shift towards towns instead.

As for how much it's worth? As a rough estimate, I'd say 2 essence = 3 of any combination of grain and materials, but that's going to vary depending on my own needs and what my game strategy is.

Essence for the most part - added flexibility

Very much depends on what you already have.  If you already have a 3 or 4 essence main fortress, it almost doesn't matter.   if its the best essence you've found and you want to toss on some unit enchantments - i'd rather have the slower build of more powerful units.

I'd rather have a 3/3/3 (I usually go enchanter) than a 2/6/1 for the vast majority of cases even if it might be less optimal.

I forgot to add that I consider forests to work as something like a 50-100% materials bonus, since it allows you to double up on production buildings which have similar production-per-material bonuses. So I'd rate your 2/6/0 with forest access as something between a 2/9/0 or a 2/12/0 rather than a 2/6/0. River access is more of a gold and food bonus, which matters more for Towns than for Conclaves or Fortresses.

Since you said that the 2/6/0 is "in a forest" and that the 3/3/2 is "just next to it", the 3/3/2 probably has forest access. In that case, I would be more likely to take the 3/3/2 than the 2/6/0 unless I'm looking to build a Town, as the two sites would then be roughly a 2/9-12/0 vs a 3-6/5-12/0-2. If the 3/3/2 does not have forest access, then which site I favor depends much more on what I'm looking to build.

Regardless, though, the value of essence as compared to grain and materials is highly variable depending on what else I have in my empire and what my game strategy is.

Depends on what the other spots that I'd be giving up have. I've passed up 3/3/3 sites for a couple 1/6/0 or 4/2/0 sites before when I'm more interested in money, production, or map control, and I've also passed up taking multiple halfway decent sites for one really good site. There isn't just one answer to these questions you're asking - multiple x/y/0 sites are probably better for your gold income than a single a/b/3 site (although with the reversion of the Propaganda nerf, that might no longer be the case), and having lots of towns floating around can let you cut back on the infrastructure of your Fortresses and Conclaves quite a bit, which is useful since the less time you need to spend building production facilities and food structures, the more time you could be training troops or producing research or something else, and you can start on that earlier than otherwise. On the other hand, if I need a troop-producing fortress, I'd take an a/b/3 before an x/y/0, and I'd rather have at least one essence on any Conclave I build (although that can be added by an Oracle, so natural essence isn't essential, unless I really want a Scroll Scribe or Archivist).

To me, 1 essence is more or less worth 1 grain or 1 material, 2 essence is worth 3 of some combination of grain and materials, 3 essence is probably worth 5 total grain and materials, and a 4+ essence site I'd almost take regardless of anything else in the area because it's a beautiful site for a troop-training fortress and because I see such sites so rarely in my games. It still depends heavily on what else is in the area, what else I have in my empire, and how I'm playing that particular game, though, so while these reflect how I usually decide on cities, they don't reflect how I always choose my cities, nor do they reflect how I always play, because sometimes it's fun to go for empires with lots of cities, and sometimes it's fun to go for empires with really good cities, and sometimes it's fun to go somewhere in between. I'd rather not get to a point where I do have a hard-and-fast rule for picking city sites, because then I'd probably end up playing the same way every game, and that would get to be boring.

Because I tend to be a lategamer, I seriously prioritize essence. It's important to remember that essence gets exponentially more useful the more of it you have, because each individual essence gets better. It's also much easier to compensate for a lack of grain or material than for a lack of essence - there are a lot of resources, techs and buildings that improve food/mat yield, but as far as I remember there are 2 things in the game that improve essence. The scrying pool is only available to enchanters factions and the oracle is a conclave level up option, so if you're not playing enchanters, it's impossible to improve essence on towns/forts.

Hmm, if you start with life magic, and you should (heal is just too powerful to forego at the start), then with Kingdom, at least as Pariden you start with +1 mat spell, +2 Growth Spell, and +1 research per essence spells. So, you can see, three extremely useful spells right as you start the game. So, 2-3 essences is huge. I don't know if Pariden gets a special spell there or its all life magic, but that grouping of spells, none of which cost much to cast destroys any other tile.