Most of what I see sitting around my neck of the woods are on farms, and both the '51 3/4 ton and '52 1/2 ton parts trucks I have are 3 window. Purchased originally by some farmer who probably figured that the 3 window was cheaper, and that the less glass you have that can get broken, the better.
I've had the pleasure of meeting at least three guys who bought an AD brand new from the dealer and their take on it seemed to be that the 5 window cab sure was slick, but with the money you save with getting the 3 window, you could buy that much more feed.
So my guess is that trucks that were going to spend the majority of their life working on a farm or ranch with occasional trips to town would be less likely to be purchased with the 5 window option than trucks that were going to be used in-town more frequently.
So while my gut tells me there were probably more 3 windows produced, it seems like it's the 5 windows that get saved the most often. I'd bet if you could do a headcount of every AD cab that still exsists in North America, you'd find the 5 window cabs outnumber (or come close to outnumbering) the 3 window cabs.
