If it's modern (fibre) cable from Virgin then I'd probably go with that if I could get a reasonable price for installation and rental. Their speeds are way higher than phone based broadband (up to 100meg now) and they don't currently seem to have too many capacity problems in their network.
The only reason for going with phone-line based broadband would be if they had got 'fibre to the cabinet' (such as BT Infinity) and you lived right next to the green distribution box - then you'd get fast (24meg) broadband without the cost of installing cable.
Whatever you get, the key to cost effectiveness is to look at 'bundle' deals (phone/tv/broadband) and not get tied into more than 12months contract - that way you can negotiate again each year.



