I have the second gen spot. I have this one and personally think it is the best (thats why i got it).
http://www.rei.com/product/815753/acr-electronics-resqlink-406-gps-personal-locator-beacon
I have not used it in an emergency... thank god... but from everything i have read a plb gets you rescued faster
Check this article out for some clarification
http://www.rei.com/learn/expert-advice/personal-locator-beacons.html