I have and use both those Eheim filters, the 2215 moves alot more water and filters mechanically better than a 2236, but the 2236 is alot easier to prime, holds more bio-media and preform maintenance with..
I would not personally use a 2236 on on anything bigger than a 20gal if that is the only filter, and then I would still add another small powerhead or koralia 1 for water movement. Atm the two Ecco 2236's I own, one is on a 20gal tall with an aquaclear 200(new model number, I think is called the 30 now), and the other one is on a 10gal with a betta/snail tank. And on the 10gal its the perfect solo filter for it but it does sorta of beat up the plants on the outlet side of the tank.