It may not be as true anymore I suppose, but I saw specific examples of hotels that were 2* or 2.5* on Priceline being 3* on Hotwire as well as Priceline 3* that were 4* on Hotwire. Some shuffling on the part of both outfits may have evened this a bit, but it still exists to a degree.
Whether or not specific hotels are worthy of either site's ratings is a separate question. I'm just thinking of the difference between the two sites. Quick example I just found on Better Bidding: in DC the Hilton Washington is 3* on Priceline and 4* on Hotwire. This sort of disparity has been around for years. But it's not universal, either. Looks like the Omni Shoreham is 4* on both.
You just have to know what you're getting, that's all. Also, even if you accept the ratings as equal, you should still be able to beat Hotwire's price with a Priceline bid in all but the rarest cases. (In the case of $97 for 4* in downtown SF, I think that can be beat.) Hotwire can also be a useful guide for prices to bid under, but sometimes it's totally out of whack (such as last week in Baltimore for me).
Also, for a number of large cities you can't count out a site like QuikBook
http://www.quikbook.com/ which can sometimes have a price good enough that I don't bother with Priceline. It'll never be cheaper than Priceline/Hotwire, but it might be a small enough difference that picking the exact hotel becomes worth it, depending upon your particular needs.