Who would you start in this situation?

I'd throw them both in the same backfield and rotate plays between them. You could get really creative with working running plays in for the more mobile QB and it opens up a whole new arena for trick plays like throwback pes or wildcat formations. You also make the mobile QB the holder on kicks so that trick plays are possible on special teams as well. If you don't want to overload them, you work a specific group of packages for each, and expand it as they become more comfortable. Or, you could just convert the mobile QB to running back and work in more liberal use of the halfback p. Either way, it's a nice problem to have.