I left SPOT because I felt that I couldn't trust it. There was one area that I traveled frequently and SPOT consistently lost me for about an hour. Maybe that was a small sample to make a decision on, but IMHO it was enough to lose me as a customer.
APRS has been amazing to me. As long as you have someone that cares watching you, it is likely to be more accurate. Personally, this group with support from people like Sam, made it possible for me to fulfill a lifelong dream of having a Ham license.
There is no perfect solution, but this is as you say at the very least an interesting technical exercise.
The problem for you Glen, is you live where there are way more crabs and waterfowl than iGates.