Outdoor Gear Lab had a review of satellite beacons and messenger devices not long ago. They covered the pros and cons of a variety of devices on the market. In my opinion, it comes down to 2 choices.

For the absolutely most reliable SOS function, get the ACR ResQLink PLB. Basically this is purely an emergency device.
Pros:
Most robust SOS function
Low cost (relatively)
No subscription fees
Cons:
Only sends "SOS" message (you can't provide any details of your predicament)
No 2 way communication.


For a good SOS function, plus 2 way comms, plus GPS mapping navigation get the Garmin InReach Explorer. This device can serve other functions besides calling for help in an emergency.
Pros:
Reliable SOS function
Compose and send messages (either routine or emergency)
Receive messages
Send tracks to family and friends
GPS mapping function (get by without a second dedicated GPS)
Cons:
Very expensive (about twice the cost of a ResQLink)
Requires paying for an annual subscription (variety of plans available)
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz