One of the things I don't hear discussed is the fact that a PLB has a much stronger signal and is more likely to get a signal out. Also, there are a lot more satellites in the PLB network than in the network that a SPOT uses, which means that there is a better chance of your signal being detected. Also, SPOT is a private company whereas a PLB is run by an international consortium of governments (US, Russia, etc.). SPOT might not be here in 2 years, but a PLB is pretty much here to stay. A SPOT must get a GPS lock in order for your position to be determined. A PLB can use doppler-shift to determine your position. Lastly, a PLB has a built in homing beacon that operates on a secondary frequency (121.5 MHz). As SAR approaches your position, they can use the secondary frequency to zero in on you.
SPOT of course has all the nice levels of messaging. Can't beat the SPOT for non-emergency use.