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As mentioned it depends largely on which network offers best performance in the places that matter to you.

Personally my O2 phone stays on 3G more often than my EE phone, but I know O2 have better coverage in and around the areas I frequent and I know why.

However as much as it can sometimes "seem" worse a lot if it comes down to how each network tunes the handover parameters that define when you switch from 3G to 2G and vice versa. It's hard to know reliably whether a network has 3G coverage simply from your phone displaying GPRS; it's more reliable to put your phone in 3G only mode. Even then O2 can be deceptive as there is often a stronger 3G900 signal that isn't used until necessary, thus only extending how far indoors you can go before dropping from 1 bar to no service, without increasing the number of bars you see the rest of the time.

Nonetheless O2's 3G coverage while not the worst in the UK is considerably worse than the market leaders outside of urban areas, but it will be improving drastically over the next two years. If you're switching network based purely on signal, I would sign up to a 12-month contract tops but certainly not 24-months; by the end of 2015 O2 will have better indoor 3G and 4G coverage than EE's outdoor coverage is today.