I was with Virgin for 8 years and had to change a few months ago to Sky due to moving house and no cable cover. In both instances I have had both broadband and HD TV
As for the difference as far as my experience is concerned there is no comparison Sky is crap, as far as the TV goes its largely the same content however the user interface for looking through channels on Sky is awful, recording programmes is no where near as intuitve or as easy nor can you record as many programmes at the same time. With Virgin you carry on looking at your current programme in a window whilst in the channel viewer wheras in Sky you lose this and get that STUPID ANNOYING MUSIC in the background !!! As far as I am conerned Virgins interface is much better.
I have also signed up to the Movie channels which are in the main awful with repeated content continously but I guess to an extent it was the same with Virgin but at least you had a bigger choice of pay per view movies and a greater backlog of previously aired programmes to choose from.
As for internet their is simply no comparison, with BT lines the dreaded contention kicks in meaning at peak times the drop off in spead is very noticeable wheares with fibre I never experienced it. To make matters worse Sky insist that you have to use their crap G rated, in my case Sagem, router which performance wise isnt a par on the latest ADSL 2, N rated, noise filtered routers commercially availalbe. As far as I know they are the only provider that stipulates you cannot use any other equipment (if you do and find out how to replace it the difference is worthwhile by the way!). To me as an IT professional it was like going back to a 56k modem experience wise.
As for the late delivery of kit, missing smartcard, poor response to the intial setup of the line and constant problems during the first 6 weeks I wont comment any further!
I am not a fan of Virgin as an organistion but if they ever lay fibre in my area I will be back like a shot!