In my experience it depends on what products you are comparing...and what metrics you want to use.
You can't compare sky's DSL product to the Virgin cable one - Virgin will win nearly every time since it tops out at 50mb (100 in some places), compared to a 24mb limit for Sky's DSL. DSL is also much more succeptable to problems with BTs network (which are very very common), wheras cable just isn't, as its well, cable.
If I could get Virgin cable I would be on it like a shot - but as it is I can only get DSL, so as a result I am with sky. I've had my share of problems, but their support team is actually really good once you get through to CSR - they know what they are on about and actually understand when you tell them you have an external NTE-5 box..it's a basic thing yes but I have been really impressed with their CSR support team.
Virgin's DSL product just isn't the same, its poor from what I know with very average support, and quite expensive compared to sky.
Things will change when sky roll out their fibre network, but not expecting this until next year at the very earliest, so for now..the two are not comparable imo. If you can get cable, go for it (internet only I hasten to add - Virgin's TV "service" is still terrible!), otherwise the Sky DSL product is amongst the best in the market.