Depends on stocking I guess. I had the fluval 305 and 205 before. The pipe work is very narrow therefore if you have alot of polluting fish it will clog quickly. I opted for the fx5 which now never clogs after trying 2 x tetra tec ex1200's.
Eheim's are quality but will be obviously more expensive than a fluval. You need to spec us up on what stock you are thinking of to get more accurate advice.
Depends on stocking and decor but im sure my 2215 would keep up with any sanely stocked 33 gallon no problem. The eheim classics do a better job than the fluval 4 series with almost no bypass at all compared to the fluval. Although either would keep ammo and nitrite to 0 in a realistic tank id prefer the eheim.
Ended up going with the Fluval 305, because its what I know - I have a smaller version on a smaller tank. I have great respect for Ehiem as well, I own a few of their products.