Green leaf bonus tile - should I build a farm or a city?

Either or actually. The +3 pop can be for cities (increased pop cap) or for food (.1 food per +1 pop)

So a farm on a +3 tile would produce 1.3 food while a city would increase the pop cap by 3 plus 30% of the total (3 + number of cities multiplied by 1.3, or for one city (3+3) X 1.3 = 7.8)

With the change to farms so that bonuses now add only +0.1, it's probably more useful to put the city in that spot and grab the 30% pop cap bonus. That way you need less city tiles and can use the extras for farms, where the +1 food base from a farm is big for food now.

I didn't like the change at first, but realized after a while that in real life, farms don't really care about what's next to them, but cities do. Now I worry about farm placement only to ensure that they buffing some cities.

Add an irrigation or other farm improvement building ontop of it and terraform and plant farms all around it.