The iPlayer application on BT Vision is just awful. But you probably already knew that!
But I've read critics reviews stating that on YouView it's excellent. Quite a smooth experience, by all accounts. And very comprehensive, too.
I never got the software update when I had Vision, so never got to try it, but I know everyone hated it. It's brilliant on YouView though.
I have thought of one advantage with Sky now, just to appease the Skyaholics, you can fast forward/rewind it easily; I hate the jumped skip.