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.
Steve: It takes some care to set up the Spot so people like your mom won't get the wrong ideas. On mine, I changed the help message to something that says "I am NOT asking for help, it is just a custom message." Spot has a custom message that can be pre-programmed, and it can be set to mean something too. I do NOT have spot send emails to people, except maybe one person. Your experience shows why.
Second, SPOT should be used more frequently during the day. Lithium batteries are NOT that expensive, and they last for nearly two weeks if you use the tracking mode. My son used mine like you, sending one message occasionally, and his experience was the same: he turned it on, sent a signal and turned it off. The units need to be left running AT LEAST 30 minutes to ensure a signal reaches a satellite. If there are any obstructions like high canyon walls, or trees in a forest, then leaving it on much longer (hours) is necessary.
For those worried about cost, the SPOT units withOUT tracking can be used. My "poor man's" tracking mode works: Use the Help button. It will send a tracking signal every 4 minutes for an hour, then quits. Using that several times a day shows people you are moving and doing fine.
I would not hesitate to use the SPOT with an emergency situation. It's tracking/bread crumbs mode is really useful, where a PLB can ONLY be used in an emergency. SPOT's signal will get out most places, and the SOS is sent every few minutes to ensure it goes out.
Edit: Adding this: The InReach advantage is the ability to send and receive messages. But with the added cost, you have to decide.
