I'm 5'10 with a 30" inseam.

My road bike is a 21" Trek 520, and my MTB is an 18" mid 90's Nishiki Colorado. These sizes work for me. I have a bit more crotch clearance on the MTB than the 520, but the next size up would probably be too high.

At any rate, providing the standover height is OK, it's not so much the seat tube length that matters as the top tube. Seat height is easy to fix, but there are limits to what you can achive with stems.