o2 and Be use the same network - so should be merged into a single entity... although I guess you do have slightly different t&c for each.
Having been with Be, o2, Virgin Media, Sky and a few others, I can say:
1 - Both o2 and Be were the same decent service, although they occasionally fell over the odd evening every few months. Having said that, they were always honest about projected speeds.
2 - Sky (the max/unlimited one), once crappy router issues were sorted (they sent me a netgear finally) were solid and cheap, and probably my choice of adsl isp. Only issue with Sky was their door salesman arguing with me about how I could have 18mb in my house if I signed up with him, even though I'd told him several times I'd done a line check and it was max of 3mb.
3 - Virgin have been a solid 19mb for the last year, without a single days outage.. unlike their crappy v+ box, it's reliable. Although, having had a history of crappy service with NTL years ago, it took me being forced into choosing 3mb max over adsl in my area or 20mb with cable to try them again, but I'm glad I did.