I typed a whole chunk worth of information on both the games and I accidentally closed the tab. UGH.
Well I'll cover ONE thing many have not.
WC3 Battlechest has PLENTY of other HILARIOUSLY FUN games on Battle.net while Demigod only has just one game in itself.
The mod-kit in WC3 enables THOUSANDS and THOUSANDS of maps with unique gameplay.
Eg.
1. Tower defense. You build towers and defend against wave of opponent creeps. Usually fun, but when the creeps start to overwhelm you, it gets irritating.
2. RPG-based games. I've seen Maplestory RPG in it and it was hilariously stupid but fun.
3. LOTS of anime and movie adaptation into the game - These are the best. I've played stuff like Resident Evil in WC3 and it was hell fun with a good team destroying those undeads. Anime games like Anime Fight are awesome too. Had a chance on my fav. Bleach characters.
4. JUST DUMB GAMES LOL - Games like Slide Spongebob Slide. Hilariously stupid. You control spongebob and you have to slide your way through the slippery ice and avoiding enemies on the ice. Its a clicking fiesta as you slide your spongebob, and I have to say it phails because battle.net is server/client, so delay is inevitable. Still, its MINDLESS FUN. Other stuff like Tree Tag, where its virtually tag+hide and seek, where treants (tree-like creatures) have to escape from Infernals. The infernals are the ones that chase the treants and once tagged, the treants will be captured and will have to await fellow uncaptured treants to free them. Its once again, mindless fun. There are also ridiculous stuff like Riddles where they'll present you riddle questions with the hints to the answers lying on the map. You'll have to search and think real hard to find the answer.
Hundreds more different games not mentioned because it will be impossible to cover them all. But yeah, as long as the mod-kit is there, new maps are bound to be released and they're HONESTLY, all that you'll need to convince you to grab a WC3 Battlechest instead of Demigod, considering you're NOT planning to pirate WC3, where you will NOT have access to all these games as they are only present on Batlle.net, which needs CD-key authenciation, where of course, pirates don't have
The value in a WC3 Battlechest is MUCH MUCH better than Demigod, and that ALONE should prompt you to get WC3 over Demigod if you could only choose one. I know this is irrelevant to Dota vs Demigod, but if you're going to play Dota the LEGIT way (by buying the Battlechest), then you will have access to Battle.net and that is where ALL these mindlessly fun stuff are there for you to play. Dota is fun, but prolonged gaming on it would be boring. These games > Dota/Demigod IMO. Sorry to Dota/Demigod fans.
Cheers.