I have a nylon string and 12 string in the "classic" shape and, I will admit mostly for reasons of variety, have a steel string with the cutaway. It makes a nice bit of "uniqueness" to my little family of guitars.

I don't want to get into an argument of physics (discussion, definitely), but does that tiny little bit of air up on the shoulder really make that much difference in sound? Luthiers are experimenting all the time with sound hole placement, and there is more than one maker that cuts a hole on the side of the guitar that faces the player. That way there is more sound directed up at the player, but according to the reviews, it really doesn't affect the sound coming out of the sound hole directly under the strings. (Anyone have any experience with one of those guitars???)

Anyway, I'm not a physicist, but it seems to me that the loss of the vibrations so far away from the sound hole, and so close to the side of the guitar would be minimal if at all. But then, like I said the reason I chose the cutaway was more aesthetics than sound.

Freightdawg