Captain rig to the rescue! First questions first...
1. What are you going to use the unit for?
2. If a dedicated boat model...how much is your spending limit?
3. If a handheld...same as 2...
some issues to consider, waas vs. dgps(they have waas in some handhelds now) type of chartkits for that model(some are downloadable ect.)
just like spearguns, when dealing with electronics what you want might not be what you need and what you need might be the opposite of what you want. For example, although I have a great northstar waas gps(which is unbelievable BTW) I still relly heavily on my Loran C(si-tex) simply b/c boats(charter) in my area have used them so long that if you tell me a loran line reading over the radio i pretty much know exactly where you are and the rec. boats have no clue what we are talking about, very handy on slow trolling days.
Back to your q...compare feature to prices. Get the one that suits your requirements and go diving! There aren't any generic gps manufacturer's out there so besides brand loyalty and personal preference there isn't much difference. Go to the store and compare the ones you are considering side by side...breeze through the instruction book...if the book is beyond comprehension, move on.