I have found O2 to be an ideal mobile phone provider and have been with them since their inception. Mine now is just a "pay and go" involvement now I am retired, but I do miss the "hook the duck" monthly game.
O2 is arguably one of the best voice/text networks and has very good coverage of rural areas, partly because of Government grants to ensure coverage for those areas.
I believe Vodafone site shares in many of these places, so Vodafone should be about equal in this regard.
But that's 2G only. Fine for voice, fine for text, great for battery life.. but pretty awful if you want mobile data. And that's where O2 falls down, big time. It has the worst 3G coverage by far, and it does make me rather fearful as to how it will roll out 4G.
As I've said before, O2 was nearly fined heavily for not reaching targets as part of its licence. It narrowly scraped through by claiming EDGE to be a 3G standard. To this day, it still has the least coverage and data speeds that are far slower (I am not aware of O2 rolling out the latest DC-HSPA standard that gives people near-4G speeds, as now offered by Three almost everywhere, EE in a very large area and Vodafone to a lesser degree - mainly cities).
I've disabled 2G to save battery (yes, 2G is better than 3G - but I save by not having the phone swapping around) and ensure that I get 3G data at all times. Many phones seem quite happy to find 2G then stick with it even though 3G is now back, and data over 2G is painful. In fact, often it doesn't work properly at all.
Vodafone is now 'simplifying' its PAYG tariffs by going from per second billing to per minute billing, which is nice. Vodafone has always been more expensive, seemingly trying to tell business users that it's like Stella - reassuringly expensive. In some way, people believe it to then have the best network and the best support. The reality can be quite different.
As the EU clamps down more on roaming, you can be sure that the likes of Vodafone will find more ways to increase revenue from people staying in the UK. I also believe the first thing you'll see (look at Vodafone's advertising) is that your mobile data bundles will be marked as 'UK only'. Why? So that when you roam, and can't be charged any more for data than at home, you'll now pay a per-MB rate that will be high in the UK too. Of course, you don't pay that rate as you have a bundle.. but, hey, UK-only..
I am sure other networks will follow Vodafone too.