I agree with reefs.... I did not realize you had decided to use a sump, but the advantages are awesome. You will be very glad you did. To give you further options, I am also impressed with this Marineland skimmer Marineland Pro In Sump Protein Skimmer 100 Marineland Pro Skimmer
, and although it would not be my first choice, if money is a huge issue, the ASM Mini G would be the absolute bottom limit that i'd consider for a 55 gallon tank Mini G Series Skimmer | Venturi Models | Protein Skimmers | Aquarium - ThatPetPlace.com
Keep in mind, one advantage to paying an extra $30 to $50 now is that you will be able to upgrade tank size to a 75 or even 90 gallon tank later without buying a new skimmer. It is a known fact that 110% of first time saltwater fishkeepers upgrade tank sizes within the first 10 minutes of setting up a new tank.
Additionally, spending a few extra bucks now on the best skimmer you can possibly afford will pay for itself by maintaining higher water quality. This means fewer water changes (saves $$$$ on salt mix) and less money spent on buffers and calcium supplements.