It depends where you will be sailing. If you remain within the coverage area you should get a good service. Iridium gives full planetary coverage, whereas the Spot (Globalstar) is more for land based and coastal communications, and it does not cover Asia, much of Africa and there are service caveats over parts of South America.
Here's a Spot (Globalstar) coverage map..make sure you click on the phone tab at the top of the map, as the Spot Messenger and Phone coverage is significantly different.
When I bought my Iridium phone 6 months ago, I was made aware through research on the net, that Globalstar units took longer to aquire a satellite, and that if the aerial was moved around a lot, as it would on the deck of a small boat in a big sea, there would be far more dropouts than with the iridium unit. I may be wrong but I think Globalstar phone minutes are 60 seconds long in the primary service area of the phone, but in other areas, roaming costs effectively reduce the number of seconds in a minute. Iridium is the same cost whether you are on land, on MT Everest, at the north Pole or in the middle of an ocean.
Have a look here http://www.cruiserlog.com/forums/f12...ones-7214.html
for previous discussions on the subject. If you do buy the Spot, please keep us informed of it's service. Now that service plans and phones are becoming less expensive, a lot of cruisers have a strong interest in satellite phone outcomes.
General consensus would appear to show the iridium network (where handsets communicate directly with the satellite) to be still the best deal short of spending squillions on Inmarsat.