I've been raising ours on miceBut she's also had a rat or two when Maru wouldn't eat it. She's looking good too!
I think there are definitely nutritional differences between mice and rats, and I have seen rat-raised boas become little fatties, but some seem to do okay on them, you just have to feed rats on a different schedule than if you were feeding mice I think. Mice go right through her! Rats even of the same size seem to take longer, plus they have all that extra protein and all that. I'm sure they're great for putting on weight though!
I think it seems to be SOME people over feed them rats, and then SOME people underfeed them on mice. If the snake is healthy, that's what matters!