CPR bak Pak: entry level, old school. adequate for a basic set up. dual bakpak more suitable for a 55g. slightly better than your prizms and seaclones.
Octopus skimmer: better then the bakpak. best value (as is typical octopus). get the BH1000 or BH2000. OTP pumps are on the outside (best for low heat transfer) and more reliable than the Resun pumps used on the other octopus hang ons (excluding BH-800S). BH-800s has a superior pump but supposedly its over pumped for the body size (potential excessive micro bubble/foam/flow issues). If you had a larger aquarium (over 75g) that would probably be the best choice from octopus.
Deltec skimmer: which one? MCE300 WAY overpriced for what it is (has come down $30 since introduced but still too high). It uses a pinwheeled maxi-jet pump (which actual worth is $40 max: -$20 for the maxijet and $20 for the tiny pinwheel) and a small flimpsy almost nano body. I'm sure it works fine but I'm not paying for euro exchange/inflated development/D&D import mark up cost. this should cost under $200. For that money get a Warner marine hang on and get what you pay for. the MCE600 is a "real" deltec with aquabee pump and good air draw. it has the typical german/deltec mark up but your getting one of the best hang on skimmers out there and one you can use on a larger tank. resale value is also better than norm.