Filtration for a 120 gal..................

Depends on what you want in the tank. I'd go with more than one filter--either add another canister or a HOB.

Depends on how messy your fish are and how often you do water changes such as my lfs uses power heads connected to an undergravel filter because the oscars are messy. The best filter for that size of tank is undergravel and canister filters combined but the power heads are expensive and if you want good bio filtration you can try a fluidized bed filter