Pretty sure that Globalstar(SPOT) uses the same satellites (GEO) that the XM radio ones use. Anyone who has used sat radio knows that when you get near a south-facing slope, it just won't work. With Iridium (LEO) the worst-case scenario is you are waiting a few minutes for a satellite to travel above you for your message to send. Id rather wait a few minutes than not being able to send anything at all with SPOT. Where we snowmobile, if you were stranded in a creek it would take hours if not days to hike up a slope in waist-deep powder.
If you are in the mid of Saskatchewan, on the water, etc GEO they may be a better option, if you are in the mountains LEO all the way