EE are getting better for solid coverage these days, not sure what its like once you go of the beaten track though. One of the things Vodafone have always been good at in my book.
Never used Giff Gaff, (O2) but know a few that have, and most have moved on now due to internet issues.
Wouldn't have thought EE and Vodafone are much difference in price when it comes to Sim Only deals at the moment.
How long you been with Voda?, give them a bashing for a good deal, forget customers services, you need to get through to retention team, can only be done by selecting option 'if your thinking of leaving us' ask for PUK Code and say your leaving because of better deals to had elsewhere, they normally offer something for you to stay.
Ive been with Voda for years, was on 50%, then 67.5% Airtime Discount, before I went Sim Only with them, good for the first 12 months inc Discount, then it all went wrong due to them not discounting Sim Only any more (15% max).
In the end signed up for a 24 month deal with really cheap and crap phone, which in effect, was a 24 month Sim Only deal for me as outed the phone on eBay for 50 notes, plus side Im now back on a 87% Airtime Discount which Ive built up over the last 4 years. Ok. you probably wont be offered that deal, but you get the idea.