I'll second the vote for the ACR. I'm part of a team that did an evaluation of the SPOT for another publication (just out this week) and our consensus was that it had serious shortcomings, and none of us would trust our life to it at this point.

Issues included poor GPS reception and performance, erratic transmission of requests, and a very poor UI (that is, blinking LEDs that require memorization or carrying a reference card, and even then aren't particularly clear or useful, combined with less-than-ideal buttons and command sequences).