I first purchased a PLB 8 years ago and have tried to keep up on what's out there since. The most difficult environment for a PLB that I would be in was a marine environment and the need for it to be truly waterproof (many different standards here--buyer beware). I also wanted a unit that included integral GPS so that when the emergency signal was sent over the 406 MHz band to the COPASS-SARSAT satellites it would include a GPS position fix to quickly give rescue crews an exact location. In addition, most true PLB's also include a 121.5 MHz homing frequency.
I've followed the SPOTs from day 1 and even with their latest versions, I do not have the confidence in them that I do with a true PLB. A true PLB is a much more robust emergency device than SPOTs are. Be careful not to confuse the two types of devices.
I also do not know of a true PLB that requires and ongoing 'subscription' cost. There is no doubt that the ability to communicate, even cryptically, is welcome, as can (often to sometimes) be done with the SPOT type devices, but I have read too many accounts of these types, even the latest designs, of devices either completely failing or working so slowly as to be a failure in a situation where very timely assistance is required. Just do you research very carefully.
PLBs are registered through NOAA and they keep on file your personal information and you are asked to provide several phone numbers so that, in the event of a PLB activation, most of which are false activations, they can confirm the veracity of the situation before committing considerable resources to that situation. Typically before a trip of much duration, I'll go into the NOAA website and update my file with pertinent information as to where I'll be and what I'll be doing and include additional names/phone numbers of contacts of folks that can assist NOAA in the event that my PLB was activated.
The main take away here is that a true PLB is a much more robust emergency device than some other similar devices. Just gotta weigh what's important to you and make a good choice.