No difference as far as I can see. I used to think Sky+ was better because no. 1 son's ordinary box had significantly worse PQ than my Sky+ set-up. I changed my mind when I gave him my old v1 Sky+ box and discovered that was just as bad on his TV. Route cause in this instance was the screwy picture settings on his TV.
Not in the case of Sky+ - as another poster has already stated it records the bitstream (the transmitted data). This is then passed through the MPEG decoder when you play it - ergo it is exactly the same quality as if you were watching it 'live'.
I thought that the picture on my Sky+ box looked slightly more washed out than my old Pace Digibox when I first installed it, but a quick fiddle with the contrast/brightness on the telly soon sorted that out.
However, on the issue of degradation there will be none due to the fact that you are watching the original bitstream. That's one of the main reasons for getting a Sky+ instead of a HDD/DVD recorder hooked up to a regular digibox.