A cheap pre-paid is a good backup especially if you are travelling to an area where you won’t have coverage with your regular phone. Consider getting some kind of waterproof cover too and think about charging options (eg car charger etc).
Be sure and leave the number with the folk at home so that in a non-emergency you can call them (and have them call you back) so you don’t run out of credit; or you can have them make an emergency call on your behalf …

I have heard of people just caring a handset with no sim (or an expired sim) and relying on the requirement of all telcos to accept an emergency call (You should check in your part of the world if this is true). I’m not sure that I’d be comfortable with that especially when you consider the “low” price of a pre-paid!

Probably most of our emergencies are really inconveniences, so being able to make a call to a non-emergency contact (a ranger or family at home) would be a good idea. So I’d get a pre-paid.

Regarding a SPOT, these are great for (remote) tracking your progress or for a real emergency, however they are one-way communication. If you do set one off it can still be hours (or days – esp in bad weather) before help can reach you. If you did have phone coverage, you can get information about how long before rescue reaches you, and you can receive instructions about what to do.

I can speak from experience with Iridium based sat phones in Australia, getting a signal can take some time. Sometimes there is no problem at all. On occasions I’ve failed to get a signal, tried an hour or two later and managed to get signal (because a satellite have come over the horizon). Once you get a signal you try not to move the phone, complete your business and hang up ! I’m not sure that the other sat companies (or handsets) are necessarily any better (at least in my part of the world). Sat phones tend to be expensive too !

Any satellite communication (SPOT or phone) will be hampered by steep terrain, so if you’re in a canyon or deep valley you may not be able to get a signal out – but I guess that goes for a regular cell phone too !