The one I carry is Ocean Signal RescueMe PLB1. It is a one-off purchase: no subscription fee and a (non-replaceable) battery that lasts 7 years. The only thing it can do is send an SOS signal to emergency services and nothing else, but that is all I need.
So at the £190 I paid for it, it is basically an "insurance" that costs me £190/7=£27 per year to give me an improved chance of survival, provided that I can still press the "Rescue Me" button. For pure rescue the sytem it uses is the most reliable system out there because it uses a dedicated search&rescue satellite network and with the received signal alone they can already triangulate your position, in case the built-in GPS doesn't manage to lock. This is not possible with systems based on satellite phone.
An the other hand, systems that regularly transmit your position (these require subscription) can be better should you get an accident where you are not able to press an SOS button. They can then go to the last known position and start searching from there.