There have been documented failures of both Spot and InReach systems tofail to reach proper agencies, with loss of life. I would not depend on them for this. Again, your choice. The spot is great for tracking--maybe not as good as the DeLorme for messaging.
The PLB works just like the 406 EPIRB. Difference is potentially shorter battery transmit life. (24 hours PLLB vs 48 Hours for EPRIB). Most EPRIB float, many self deploy, and most have the antenna pointed in appropraite position.
Even the floating PLB often require the instrument to be held in a position where the antenna isi pointing skyward.
The rated shelf life on most PLB is 5 years (there are a couple slightly longer) The real life battery life is closer to 10 years. But, we choose to have a in service unit as our primary.
The PLB is registered to a person--thus useful when hiking, skiing, kayaking etc--even driving the RV in remote areas. The EPIRB is registered to a specific vessel. Thus is a bit more limited.
The big feature of the PLB is it can be easily worn on your person--in the dinghy, ashore etc.
You may have a qualified captain, when that heart attack occurs, but is she capable of giving CPR, defibrillating and also driving the boat back to safety at a speed that will save your neck? Unlikely.
There are cases of PLB being used in inland waters which many of us use, for medical emergencies--not just boats lost at sea. I sure hope that I will never have to use my PLB--or the water proof hand held radio (now with DSC)---but it migh save my or my wife's life. A couple of hundred bucks, is a tank of fuel--give or take...
There may not be current avaialable AIS /EPRIB system, but I am sure there are several in the works, and shoud be available shortly.
There are a number of MOB AIS systems, so you can spend an extra $200 and get one of thoes Harvey, if that is a goal you have. These are small and woud fit will with the Smart Find (one is made by McMurdo.