Currently looking at switching from Vodafone to EE but have exactly the same questions you have.
What I've done is bought (well got one free from their website) an EE pay as you talk sim. Popped a bundle onto it so I can test the coverage and data speeds.
I'd probably recommend doing the same, just to check you're seeing full signal in the areas you live and travel to. Just means you get a risk free 'try before you switch' experience.
