EH!?
a) You mean your phone decision is based on coverage for one particular caving location on Mendip? You must visit this place a lot or live in the hedge nearby in which case....
b) Surely you know it so well there's no chance of you having an unexpected accident requiring a telephone callout?
c) Your question presumes that someone has a lot of different types of phone who sits in laybys on Mendip checking to see which one has the greatest number of aerial bars showing up on the display, keeping a record of this information and then trawling through internet forums to see whether anyone wants to know about such stuff. This seems mildly far-fetched and massively anal to me so you're only likely to get answers from people who own just the one phone and who have infrequently needed to make a call from the layby near GB and I imagine that pretty nearly all of whom will say stuff like "My phone with so-and-so seems to work OK on Mendip"; this isn't giving you a definitive which-is-better-than-the-other comparison which you appear to be after.
d) bring on the responses....
Please don't think I'm being rude BTW; it just seems like a strange question to which I doubt if anyone can give you an accurate, useful and reliable response. But, hey, that's tempting fate and I bet someone will now respond with copious details including grid references, frequencies, relative clarity of signal, gaussian blurring, squelch filtering etc.. (p.s. I've made some of this up
)