What do you want to do with the remote? What electronics do you want to control with it?
They all do the same basic tasks but there a few additional features when you move higher up in the models (compare some at logitech.com/harmony to see and learn about the differences). There used to be a walkthrough of how the remote works on that website but I am having trouble finding it.
As for the price you can normally get them for less on Ebay. Also look in the Deals forum, I have seen members post some deals for Harmony remotes there before.