I've got a Fluval 404 on a 29 gallon, and it does produce a decent current up top (I have the outlet in one corner aimed straight down the back of the tank), but the bottom 2/3's of the tank is still fairly calm..... some of my fish, particularly the rummynose tetras, love going up & surfing the current.
I had a 204 on it, and it was able to keep up with the bio-load just fine, but I plan on trying my hand at some plants in the near future, so I wanted the extra power because I know some flow is going to be lost when I run a CO2 reactor inline.