I have a Fluval C3 on my 29 gallon and it works great.
I too am replacing Aqua-Tech (30-60 -330GPH) on the 38 gallon but I am going with 2 aquaclear 50's. Aquaclears are a wee bit easier to do custom media. I am going with a two filter system as a fail safe and because space behind the tank to the wall is limited.
The aquaclear also comes with an extra extension tube to go deep into the tank. The Fluval C3 you have to order it extra.
For tips on making these filters give you some really crystal clear & polished water, go here
I want me water even cleaner, any suggestion. Pictures inside!
Also, for either filter - be sure and order the Fluval Pre Filter Sponge.