Do you need Text to voice so that the gps tells you what street name to turn on, do you need bluetooth connectivity, do you want real time traffic info?
How big of a unit do you want, do you want a wide screen unit? Most importantly, what price range are you looking at?
They all have pretty much every other feature. I would read this guide so you know what you are talking about: http://tinyurl.com/yrequb