In my opinion bare bottom is the worst. Yes, it may be easier to clean but it is unnatural to the animal and looks terrible. Now when it comes to sand vs gravel there is pro's and con's to both. I have seen that stingrays tend to enjoy sand more and be happier with it and do better with it, especially saltwater rays (not so much freshwater as thier environments often have a gravel bottom) but sand also is hard to clean and can very often grow bad bacteria that can be harmful. With gravel, it is personally the best for rays in my opinion, but only very tiny pieces, not large ones and try and get htem as soft as possible because the edges can cut the rays. Larger gravel makes it harder for them to bury under too. Gravel is very easy to clean also using a gravel vac, therefore making the tank cleaner and easier to maintain. So the best bet would be a very fine gravel that you are willing to replace on occasion cause even with a gravel vac you will suck up part of the gravel.
But thats just my opinion and I dont consider myself an expert at fish, just speaking from experience.