people use fx5's on 55g tanks all the time, only if its not too much current for your fish though.
as for other canisters, there are lots of sizes availible. remember that on most boxes the GPH rating is usually without media and head. one company puts the true GPH on the box but i cant remember which.
the tank rating is pretty pants too, i would look for a filter that is rated to double your tank volume. for example i wouldnt use a fluval 405 (rated up to 400L) on more than a 200 litre tank. the flow just isnt enough IMO.
eheim are generally considered the standard for the filter industry but thay lack a bit in flow for my taste and are ridiculously expensive to buy new. eheim promise to make parts for an off production filter for at least a decade after last production.
flluval generally have more bang for your buck but have had parts issues with past filter models that are only a year or two off production. though the fx5 seems to be considered the best at the moment by most except the eheim fan boys/girls. the fx5 has good flow,good media capacity, good price and it seems excellent reliability.
there are other brands too such as rena and tetra. rena seem popular on this board.
it could be worth looking at a sump or hang on back filters as well. sumps can have huge GPH and they can be cheap to set up.
good luck deciding.
