From an aerodynamic point of view, I agree that the two are identical.
However, the reason for doing them, and therefore the method of teaching them, is quite different. I recently had a student who had become very confused about the whole idea of cross-wind landings when he confused the two different types of slip. Once I broke the slip down into its two different types, and explained the whys and hows of each type, it made a lot more sense to him (and his cross-wind landings improved overnight).
I don't know of any references off the top of my head, but I'd be surprised if Langeweische didn't have something to say on the subject?
FFF
----------------