I never realized that they have a "solid" version. You have to go to page 2 and 3 of their Vic windshield pages ( Cross Country / Cross Country Tour
) in order to order that kind. (But, as you copied in your post, there is a picture of both kinds on that first page.)
As you can see in the air-flow diagram on their home page ( Home - Madstad Engineering
), air has to come underneath the windshield to make the anti-buffeting system work. I suspect, therefore, that the difference is that the solid (i.e., non-cut-out) version has to reside, at its lowest setting, farther above the deck of the fairing. Without such positioning, I don't think you'd get sufficient air flow from the bottom of it.
Put the other way, I think the bulk of cut-out version can, in its lowest position, be closer to the top of the fairing.
And put a third way, this implies to me that both systems would work, but that the buyer is offered a choice, for cosmetic and esthetic reasons.
Over the years, I've read that some folks have just the Madstad brackets, and some other brand of windshield, and that they are pleased with whatever setup they have. (This is aside from the many folks who think the brackets are fugly. Also, you can't pull this stunt with windshields with curved upper lips in them, such as the KockWerks Flare, because if you raise the windshield too high, you'll wind up looking through that curved part, which is not intended to be looked through.)
I've been using the 11" light tint Madstad windshield and brackets for almost three years now -- see attached -- and I'm pleased with it. I'm 5'9", and mostly leave it in its lowest position, slanted as much as it goes ... unless it's cold or windy or I'm doing a superslab day; in those cases, I'll raise it higher.
Incidentally, the first time I was in decent rain with the system, I moved my left hand up, down, and around, and verified that the rain flow was the same as the regular, dry, air flow.