It really depends what you want...I live in a small private estate which only has cable lines coming into it. Virgin, or Telewest, or their predecessors were cabling the town at the time the road was built. They cabled it, and BT decided not to bother. So me and my 19 neighbours have cable.
My father, in his 70s and a complete technophobe, is totally different. He lives in a private avenue and neighbours objected to the road being dug up for cable when they were going round, so he is 100% cable free...
My view on both (as my father's advisor!)...
It really depends what is important to you?
Broadband?
Cable is better - you will have closer to the advertised bandwidth. Don't worry about freebies like Home Hubs, etc. The ongoing price and bandwidth are the main thing to concentrate on...Virgin have unlimited downloads and much better bandwidth. I've never had a problem with their broadbadnd. SKY's broadband is pretty good, but over copper it still suffers a little if you are a long way from the exchange, like my father.
TV channels?
I don't have HD TV yet...Virgin don't have much choice, and I am stuck with them...so I ain't paying for such a limited choice. Sky win hands down here. If you want the latest and greatest HD movies and sports. Sky is the default choice.
Customer Service
Virgin have been just about OK with me...imagine a mobile phone company...they increase your monthly bill up out of the blue, so you phone up to cancel, and magically they bring it back down to what you were paying...that's Virgin. Don't accept price increases for a second, complain, YOU WILL GET A DISCOUNT! I don't have too much experience yet with Sky. One personal observation...Virgin's Customer Services are only open until 8pm. If, like me, that is rather early in your evening, you are stuck if you have a problem. TV channel gone down, can't see the 9 o'clock news...wait until tomorrow....
In summary?
VIrgin Broadband is great, TV is good. Sky TV is Great, broadband is good. My advice? Are you an online freak, is the internet your life...go Virgin. OR Do you want the best HD TV package and decent braodband...go Sky. Assuming you have the choice of both course. There are lots more broadband vendors, but to be honest Virgin/Sky are right up there with the best of them.