In onroad I dont think there is one more popular then the other,the whole car of the month thing doesnt happen as much on the onroad as just about any race level kit can put you on the podium, maybe you should call all the companies and try to get there sales figures if its that important to you.
go to the track youll be racing at and figure out what car is more popular there, it ends up being more of a geographical thing as some areas one car is more popular then other areas.
Dustin Colt45 Aguirre
Kyosho RB6--Futaba--HW--Shuurspeed 17.5
Kyosho SCR--Hitec--Lrp--Yeah 13.5
Durango Dex210--Futaba--Trakpower--Tacon 8.5