We had the SPOT but found it had a couple of serious deficiencies. First, due to use of geosynchronous satellites, it often cannot get the signal through - and of course there is no feedback to tell you this. It is one way so of limited use for any unusual circumstances. Poor coverage in higher latitudes.
We use a Delorme SE which uses lower orbiting satellites and so has coverage all over the world and it doesn't matter too much where in a vehicle or on your person the tracker is located. It has two way verification of tracking points and messages sent. Can also send text messages and emails to any phone number or email address in the world and they can reply.
If you do need to send an emergency call, you can clarify the circumstances using the message facility if you want to. Just this feature makes it unique as in most other personal locator beacons and SPOT, first you haven't a clue idf the emergency call is received and then the responding authority has no idea of what the problem is.
More expensive, but the extra features make it worth it.
Not that long ago two young stockmen died of thirst in the outback even though they were only 13 miles from home. SPOT units were sitting back at the homestead but they hadn't bothered to take them.