Not a lot of differences between any of them. It is just a plastic housing that holds your filters. The difference is in the cartridges. Some of them are better quality than others. Other things to look for are quick disconnects - makes for easier maintenance, a pressure valve to see if your faucet is putting out the proper pressure for optimal performance (70 PSI is what most units recommend), and a TDS monitor so you can tell when it is time to change cartridges.
I would say to make sure and get a 4 stage unit which will include, sediment, carbon block, membrane, and a final deionizing resin chamber. If you live in an area where there are a lot of silicates in the water (or if you constantly fight diatoms in your tank) get an add-on silicate remover.