I went through this a couple of months ago and came up with a decision to go with the PLB.
First off, the Microfix warranty and batteries are good for 5 years so at that point the price of a SPOT vs a Microfix is a wash. After that point the SPOT continues to cost you the service fee of $99/yr but, as we observe the pace of technological advancements in electronics and communications it's pretty likely that both of these things will look like bricks compared to what will be available five years from now.
The tracking and messaging aspects of SPOT sound pretty cool but reviews have demonstrated that they aren't that reliable and in the end, after the novelty wears off, are you really going to be sending out that many messages or tracking yourself on Google Maps? I don't think so.
It boils down to one thing for me - the absolute reliability of the instrument and the chances of it getting me found in a timely manner if the need arises. As has been mentioned the Globestar system has had some problems and you are relying on a private enterprise. What if your yearly renewal fee didn't get processed or updated? There isn't a clear answer to this question. Also the SPOT is relant on a good GPS signal as as well as a Globestar signal as it has no means of deriving your location through Doppler as does the PLB so if the gps unit or antenna fails, or your location is not good for gps signal reception you are pretty much SOL.
So that's how I made my decision but it's by no means a slam dunk for either side.