A lot of people diss the SPOT as not as good as a PLB ... that's like saying a motor home is not as good as a house. It all depends on what you want to accomplish.
A PLB can definitely be a lifesaver, ditto an installed 406 mhz ELT. But neither one will tell your significant others where you're at at a given time ... and neither one allows you to send an "I'm OK" message or at "I'm not in dire trouble but I need some assistance" message as will a Spot.
And the latest generation Spot, sold jointly with the DeLorme PN60 gps, also allows you to send text messages via Sat.
So, if you want a tool that will call in the Mounties when you're in dire trouble (or dead), the most reliable tool for that is either a 406 mhz PLB or ELT. But that's the only thing either will do.
For any other communcation, you need either a Spot or a much more expensive Sat phone.
It's not a matter of "either/or", it's a matter of what it is you want to do, and what you're willing to pay.
If you want to be totally covered, get a 406 mhz ELT for your plane, a 406 mhz PLB for your person, and a Spot, and a Sat phone. However, that's getting pretty expensive.