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)