Android 4.0 (Ice Cream Sandwich) is the most polished version to date.  As far as hardware I highly recommend the Galaxy Nexus.  It is one of only a few device already running 4.0, and it will be the first device to get updates directly from Google.   Also has NFC and a large, sexy screen for showing off the Bitcoin clients.

In regard to service, I'm a little surprised Verizon doesn't offer the best signal out there where the barns are located.  Your Walmart Tracphone is likely piggybacking off Sprint's CDMA network, so you might want to consider Sprint also.

In regard to switching out devices, you can't beat AT&T and T-Mobile (both GSM networks).  Switching phones is as easy as moving your SIM card.  And all the cool European/Asian import phones tend to be GSM. (T-Mobile USA doesn't offer the Galaxy Nexus, so I bought an unlocked GSM model from the UK)