I'm pretty sure Red Tailed Sharks are less aggressive, but I don't think either would go with platies, cause of the bright colors. While the shark is small, it might work, but once he gets to 5 inches, forget it.
No platys need fairly agreesive fish like barbs and such to go with them sharks.
I prefer the red tailed shark more striking looking.
How many gallons or litres is the tank.
Its been my experience that Red Tail Sharks are definantly more agressive than Rainbow Sharks. That said, as adults RTS in my opinion are also a much better looking fish as their body's are jet black as opposed to a rainbow whose body is definantly more greyish. RTS are also a little thicker than Rainbow's.