Actually. I went from PT to Reaper. Though I've only been using PT for two and a half year and may not been the most pro in it. I can't afford Pro Tools anymore, so I went over to Reaper.
Reaper is optimalized for Windows, while PT for Mac.
Reaper uses several plug-ins formats, VST for instance; alot of these around the internet for free.
Pro Tools uses Rtas and the plugins costs a fortune.
If you can afford Pro Tools and uses mac you should go for it, but if you don't can afford PT/uses windows you should go for Reaper.
The sound quality is basicly the same. To youre question again; If you get the best PT mix you can get and the best Reaper mix you can get, it's hard to notice the difference. I would say impossible unless you're a robot. Aka they are the same for normal users.