mountain bike shoes can be bought with various degrees of sole stiffness. IMO, there is no advantage given to road shoes WRT to stiffness. remember there are professional MTB cyclists too and they wouldn't put up with anything less. the size and shape of a cleat is irrelevant as long as the sole has adequate stiffness.

BTW, i prefer Eggbeaters to SPDs for just about everything.

so it's Eggbeaters and SIDI Dominators for me.