Canisters have the potential to handle a larger biological load because they hold more media, and some have slower water movement that also helps. But you need to put it into perspective. I've never seen a hanging filter that can't handle the biological load of a tank, and if your ammonia and nitrite are at 0 where they should be (assuming the tank is cycled) then you're basically spending your hard earned money to fix something that's not broken in the first place.
However, if you really want a canister, just run it at the same time as the hanging filter for a couple of weeks to establish a good bacteria colony in it, and then you can remove the old filter (or you can always run both if it's not too much water movement). When you start shopping for them you'll quickly know which model suits you - probably the smallest one of any brand.
