Sky's Broadband is slower, but also dirt cheap compared to Virgin's. Also, I think Sky have more HD channels.
I work out Virgin's ultimate package (VIP package, telly, phone & BB) as £93.25 + £13.90 line rental = £107.15
Sky's top package (also telly, phone & BB) is £73.25 + £14.50 line rental = £87.75
So Sky is probably cheaper than Virgin on the whole, too. Although Virgin's introductory offers might offset that extra cost.