I would definitely stick with NTL.
There are extra channels on Sky - but none which you would waste the bandwidth on.
Most importantly THERE IS NO RED DOT ON NTL
The 'interactive services' are mainly cr@p to seperate fools from their money- and the red dot ruins the picture --- yes I know you can press back up --- not when its a video you can't. If you have not had Sky you cannot estimate how irritating it is. See
http://dvdfever.co.uk/pressrel/pressred.shtml for more details
Similarly Sky+ is overhyped junk designed to lock you into Rupert cos you have bought kit which only works on sky (by another PVR)
NTL is a little cheaper if you have all 3 services CF sky. Customer service is way better and did I mention NO RED DOT