I think, honestly, it depends on what you want the device to do for you. For me at least, they are different tools for different jobs. yeah the spot is supposed to send a signal out that you need help but its main appeal is sending out "ok" messages and ability to track.
So you have to ask yourself what would put your family at ease more?
If carrying an item just for that extreme emergency, then the beacon would probably be better since its a one time fee and should work no matter the conditions.
If you want to send updates if not track logs and the ability to call for help, then the spot would be the way to go.

You may want to ask around and see if anyone has used the spot in the area you plan to hike and how the service was. That might make up your mind for you.