woah, this could open up a can of worms. Cue ALwin with a long post?
Picture quality is a very broad term, and will be dependent on a lot of things really. Don't let the megapixel race fool you in to thinking it will give you great pictures, but also don't ignore it completely.
The lens that is being used will possibly have the biggest effect on the image quality, but for under £500 im sure you'd be using the kit lens, or a 50mm prime (which gives excellent images).
To be honest i think all of the beginner/mid-level DSLRs that you can get for around £500, whether new or second hand would produce comparable images. Different situations may be suited to different cameras or lenses, so there are just too many factors really. Unfortunately such things are not so black or white.