As Catwoman said, a SPOT Messenger is a satellite messenger. It requires you to pay an an annual subscription to use the satellite service. The SPOT is something that will help you if you get lost or need help on the mountain and don't have cell coverage. A SPOT will not save your bacon if you get buried in an avalanche.
All avalanche beacons since the late 1990's operate on the same frequency. It does not necessarily matter which beacon you get as they all work together. What is important is that you learn how to use it and learn it well. The BCA Tracker is very easy to use, the Tracker II is even easier to use. Other beacons have their pros as well.
Hopefully that helps.
Cade