There's less on Sky, but it's newer programmes and HD. It's really only a catch up service though, iPlayer isn't the full service either.
iplayer is far better quality on sky. I haven't noticed anything missing on there.
ITV player is far more comprehensive on sky.
Why do you say there is less on sky? You and I both know there is far more. I can't think of anything that is available on BT that isn't available on sky, except 4od.