I wish I would have went with barebottom for ease of maintenance.. I have to syphon poop every day it seems. Sometimes twice a day.
If I had a barebottom with a pre-filter I could just change the pre-filter once a day.
When Rays breed they release enermous amounts of body fluids so I would imagine having the more pristine environment would be paramount.
I find that I get a bits of waste and particles in my sand, and I have to syphon 72x24 inches every day or two as well.. I tried to allow for more current towards the bottom of the tank to push those particles into the overflow/canister, but the rays didn't seem to like it. You don't need a very strong current to move all the waste in a barebottom tank to one location.
I would say sand for stress release of juveniles.. Also if your looking to make a community type display setup.. but for functionality of breeding rays, barebottom all the way..