i've got a red tail, currently in a 10 gallon( upgrading to 20 soon) with 2 tetras, 2 barbs and 2 platies, they get along extremely well, there was a bit of chasing at first, but now they swim side by side, even when feeding, which i have read is unusual for the red tail, but i beleive that the only noticable difference between a red tail and rainbow is the fin colouration, the rainbow having all red-orange fins
red tails are very territorial they need a cave to hide in a defend, if the red tail gets along with the tank mates extremely well, it may even share the cave, in my tank his cave is half a coconut shell, it has a small hole in the top of it that he likes to swim through all the time as he is still small enough to fit, but i beleive that all sharks are semi aggresive and will defend they're territory, so either have a larger tank, or if you're limited, try to have as many hiding spots as possible for other fish
As the other guys have said, it comes down to what you like best. I like them both, though the red tail was the boss of my rainbows, but the rainbows constantly fought, which was awesome to watch, & fine for them.
I'd prefer a rainbow for your tank, I think they're generally less aggressive.