In reply to gilliesp:
They do indeed share masts. There are some differences in coverage in urban areas but in rural areas I would expect the coverage to be very similar as the kit will generally be co-located. There could still be differences at the fringes due to the frequencies used. I'm not an expert in this area however.
Of course, for emergencies your phone will roam to any network it can find, so you don't need to worry about that aspect.
If you're saving money by switching, I'd probably go for it.
Disclosure: I work for a major mobile company, but the above is my personal opinion.