I'm not a fan of Fluval's U series either. They take up so much space and end up being a distraction.
The Duettos are my favorite. Slim, quiet, and the media chambers are separated from the inlet, so when you stop/take out the filter, all the gunk doesn't end up falling out of the filter. The only negative I've found is that it clogs easier.
I use a Cascade 300 as well, but it does suffer from that "gunk falls out" problem. It also (strangely enough) doesn't force the water to go through the sponge, but I suppose that's only an issue if you need fine filtration.