1) The E71x has AT&T Navigator for subscription turn by turn directions, rather than Nokia Maps. It also has CV streaming video content provided by AT&T and included in the unlimited data plan. CV-- probably not so important. While Nokia Maps has gotten much better, I still think AT&T Naviator (Telenav) is superior. That's about it for differences in that department.
2) AT&T requires a smartphone data plan with smartphones that have a QWERTY. The difference is in the billing, not in what you get for the money.
3) You can use WiFi independent of an AT&T data plan. If you do have a PDA or smartphone plan, they generally include AT&T hotspots with the data plan (most Starbucks are switching from T-Mo to AT&T hotspot service).
4) Both data plans allow you to get your POP3 email. AT&T currently does not block this, even with the feature phone plan.
5) Getting a no-contract arrangement with all the bells and whistles of a contract plan can be challenging. You may have to charm and cajole the rep (go to a corporate store, not a 3rd party wireless dealer).
6) Can't think of any.
-------------------- Lisa Gade Editor in Chief, MobileTechReview