BT Vision is currently IMO a poor relation to sky. By far. Most programming is over freeview, with Sky Sports 1/2 also available via encrypted freeview channels, and some limited on demand content delivered over IP.
AFAIK the freeview decoder isn't HD, only limited VOD is.
However BT have been investing in a content delivery network & adding multicast support which IMO puts them in a far strong position for competing with satellite to carry programming without the current capacity limits.
With "youview" arriving next year I can only assume/hope we'll see a much more compelling BT vision service in the coming year or so with far more streaming HD content, especially for fttx users.
Sky has content mostly sewn up, but does make this available to virgin (they have to), so I assume the same will apply to BT.
Note if you drop sky broadband but keep sky you may lose access to anytiime+ though rumours are that the ISP traffic block is now removed even though non-BB users can't sign up. A good sign of this being opened up.