PLB or any 406 ELT with GPS is way to go!
PLBs getting better; many different models! How good depends on your environment; over land or water etc. Some float, some don't unless you put a neoprene cover on it. Newest ones are floaters and have a built in strobe! But the best (and most expensive) for a overwater type; is a water activated one where it can stay in the survival vest pocket (below the water) since it has a 406 antennae that is around the neck incorporated around the vest and is also a voice capable radio. I think the GPS antennae is also around the neck but not sure. And all these activate the SAR system! The Spot Messenger I won't comment on since I really don't know that system..
Former SAR Helo guy and RCC Officer in Charge