I have been around the AD trucks all of my life while growing up in wheat country in western Kansas. I think that I could safely say that 80-90% of the ADs were 3 window trucks. By trucks I also mean pickups.
I now live in south central Kansas and would guess that 3 window trucks would outnumber the 5 window units 3 to 1, perhaps more.
Those ADs I spot on Ebay seem about 50/50, perhaps a little heavier on the 3 window. The 5 window trucks are far from rare which is how many Ebay 5 window trucks are described.
Even though you didn't want personal preferences it comes along with my post. I now own a 3100, a 3800, and a 6400 with the 3100 being the only 5 window truck.
Since I actually prefer the looks of the 3 window it will be a toss up whether to keep the 5 window on the 3100 with its additional safety for city driving, or swap the 3 window from the 6400 because the cab is in better condition.
I still question why there were ever 5 window cabs on grain trucks since you have to use the mirrors to see around the bed corners? Also, why deluxe cabs on farm trucks?