Well, sorting by discount is the most useful in my opinion. I use sorting by user rating when browsing steam and adding new items to my wishlist, so once the game is already on my wishlist, sorting by rating makes little sense, because I already know I want it and it's only the matter of the price. :-)
Sorting the general list of deals by user rating, now that's another story and much more useful. Generally, the more sorting options - the better, but it would be great if everyone could choose and save their defaults.