We use these devices a lot in sailplanes, some use Spot and some Delorme. I have a Delorme, and I have probably 30 friends with one or the other. The Spot is cheaper, but the Delorme is much better in every other respect. The Spot is one way communication and not always reliable. It sends a position report and hopes that it is received. It is not at all unusual for a glider with a Spot to drop many or all position reports in a day. The Delorme uses a more reliable satellite constellation, and the communication is two way, that is, it sends the position report, enquires of the satellite whether it was received, and if not continues to try to send it. Delorme reports are quite reliable. They do need "a clear view of the sky" but I have found that it works from inside my camper and inside my boat.