I love my Aquaclear 110 on my 55 gallon. Sure, it's a bit of overkill, but you can turn the flow down. I like the huge amount of filter media it has - and the flexibility of what kind of filter media you can shove into the basket. Also, it is no deeper than the Aquaclear 70, just longer. So, the tank can be just as close to the wall. Aquaclear also has a model for 50 gallon tanks (called the Aquaclear50, of course), but I think everyone should always overfilter at least a little. The ratings on filters are really assuming a pretty light load of fish - so getting a larger filter can reduce time between cleanings and give you a a bit more peace of mind.
The aquaclear filters are of the HOB variety, they hang on the back of you tank. If you want to go with a canister instead, most people say Eheim is the best brand to go with.