In my humble opinion, a portable GPS is a supplement to navigation - not the primary tool. That said - my criteria for a portable GPS is pretty basic.
Wow - I am surprised by the number of iFly responses. I personnally use the the 700 - but as far as I know, I'm the only one at my home base who does. There are lots of Aera users - not a bad choice if cost is not a factor and lots of ipad users - good choice if you are going to use it for other things. Other problems with the iFly - no battery is the big one - I have had problems with the power cord. Also, it seems to refresh fairly slowly (my actual position is a few miles ahead of what is being depicted). Not a big issue for me in that I fly a slow airplane (<150 kts).
The iFly is expandable - I plan to add ADS-B in to pick up weather.