Personally I don't carry any of these devices. My one experience using a SPOT mirrored Krishna's. On the JMT I had agreed to send an OK message every evening when I made camp. Several times the SPOT message didn't get though, making my family much more worried than if I hadn't carried the SPOT in the first place. When I later aborted my trip on day 15, I pressed the HELP (not SOS) button, which I had previously agreed with a friend would be my way of signaling an early exit. But my mother didn't know this, and when she started receiving emails every 5 minutes that said "HELP!" in all caps, she panicked and called the forest service, sheriff, and everyone else she could think of. It took me almost 24 hours to reach a phone, during which time she was worried sick. The experience convinced me that in some cases, poor communication is worse than no communication.
As I understand it, the advantage of a PLB is that when it comes to the all-important task of summoning an emergency team to save your life, it's far more likely to succeed than a SPOT or InReach. It uses a different, reportedly more reliable satellite network (COSPAS/SARSAT rather than Iridium or Globalstar), transmits to satellites with much stronger radio power (5 watts vs 1.6W on the InReach and 0.4W on the SPOT), and also emits a local area homing beacon at 121.5MHz in addition to the satellite signal. The ResQLink PLB even has a strobe light, weighs less than both InReach and SPOT, and costs less over 2-3 years when you factor in the subscription costs of the other two. Unless I'm missing something, the choice between these three seems like a no-brainer as the PLB is clearly best, unless you value messaging more highly than getting rescued in an emergency.
