SPOT is a great choice: full coverage, long battery life, and emergency support. It costs $100–150 for the unit + required $100 per year to send messages + optional $50 for continuous tracking.
Unless you are looking for continuous coverage at all times, the phone may still be a good option. I understand your concern about signal availability. For traveling on roads in the US (where coverage is expected more often than not, depending on your carrier), and at no additional cost (if you already have a smart phone), the solution with a phone works great for me.
My budget is limited so I use the iPhone in two ways:
- For family members with an Apple mobile device, they track me on 'Find My Friends' app
- For others, I use Google Latitude and they can track me on smart phones and web sites
These apps are always on, and they do not add too much drain to battery life.