i would for sure use a skimmer, especially if you plan on fish. I have the bakpak, but i have heard that the remora is much better. Either will work fine for a 20 gallon. If it were a bigger tank, I would suggest the remora because it is more efficient.
I have used both of them but prefer the Remora mostly because it is alot quieter plus I am not a fan of Rio powerheads. I have the Remora with the MaxiJet 1200. With a 20 long either skimmer will hang below the back of the tank so if the tank sits on a table where the skimmer couldn't hang below the tanks bottom then neither of these will work for you. I just wanted to mention that fact. Good luck, Skip
Everyone above has very valid points. A skimmer isnt 100% necessary in your situation but if you are going to use one I'm with Skip and reefman23.
I've used both and the main benefits to the remora over the CPR are 1) they just work better 2) better pump 3) less noise 4) its a bit smaller than the CPR and 5) the way the remora returns water makes for more surface agitation and better current.