Going by my own experiences, and those of my friends, it seems to be pot luck as to which you prefer. We were initially with Telewest before it was bought by Virgin, and it was great. Then Virgin bought it, the service went downhill, and we left to go to Sky. Sky was ok, until we asked about getting a free Sky+ box, given that we'd been with them for a while, and that was what they were offering new customers. They wouldn't even give us a discount, so we left them, and went to Virgin (who gave us a free HD+ box and a very good package).
We're still with Virgin now, and have no problems with them. My friend found it the other way round, she had endless problems after getting Virgin installed, so after 27 out of her initial 28 day period, she told them to take it all out, and got Sky instead.
It really seems to depend what deal you get! What I'd say is only sign up for 12 months, and at around 11 months, look around to see what the "Other side" is doing in terms of deals
Always useful as a weapon to renegotiate your contract