I was on rodent pros website and was reading their nutrition chart and noticed that the rats have more protein than mice, but also more fat. I think they only sized rat that you can feed a corn snake without it getting fat are the weanlings. (This is all according to what I read on the chart no other sorce told me) But most people who feed rats normally feed those medium rat that you can find in a pet store.
The problem with this is that weanling rats are already on the very large side for your average corn snake, at up to twice (or almost so) the weight of an average adult mouse (the largest most corn snakes need). Besides the fact that a higher percentage of that weight is fat, it's already too big for a great deal of corn snakes to ingest safely and easily, much less without getting very fat.
Anything larger than a weanling? Yeah, no, I can't imagine even the largest corn snakes in captivity handling anything larger than a "small" adult rat, tops. Heck, my 800 gram kisatchie corn and my roommate's old 900 gram corn--neither could safely handle more than a weanling, or slightly larger. Incidentally, both came in somewhat overweight from being fed nothing but weanling rats.
Hands down, mice over rats. At least for me. Every adult corn snake I've met fed exclusively on rats has been overweight. Not necessarily obese, but chunky enough to be concerned. Besides that, they just plain thrive on plain ol' mice... Why go to the trouble, expense, and deal with the pitfalls (that fat content, again) of juvenile rats when mice have consistent, tried-and-true success?
... I think I need to take classes on how not to be long-winded. LOL. There's my $.02, anyway.
