Network choice is very personal.
I used to be on three, but sadly had no coverage at home/work so was forced to leave. They also dropped quite a lot of their 2G coverage (roamed onto Orange) leading to rather too many gaps for my liking. That being said in coverage they had the best 3g, throughput,prices around. Ultimately though with something like a basic text message not being possible out of coverage I just had to switch. *very reluctantly*.
"3" put all their original 3G network masts into a joint venture with TMobile known as MBNL. This covered 3G only, but was same sites, same antenna but different backhaul. so coverage should be identical. Tmob/Orange then merged to form Everything everywhere, and kept the 50% in MBNL jointly. I think the Orange network is still merging but eventually 3G coverage should be similar across all 3.
I moved to vodafone - definately far less patches -- though they still exist. 3G not as good as 3, but improving. Some HSPA+ (yes the SGS2 does show a nice H+ icon). Data prices not as good and rising :-(
Vodafone have an initiative with O2 to improve coverage (cornerstone) though I think this is site share only.
O2 itself always seemed "optimistic" in their coverage predictions. My area still marked down for full bars 3g/2G, yet whenever I tried even 2G was patchy and no sight of 3G -- this pattern was repeated in other locations locally. Nowhere near as much as 3G and issues with data in general