I have both. Driving them on carpet and on 15' no ARS. Equally fast cars. ARS on carpet doesn't work for me and most guys I know. It's nice on big open asphalt tracks thou. But the 15' has some nice features as tweak free motor mount, ball bearing supported ARB's that can be adjusted from the top. On the 14' the tweak adjustment of the ARB's was just PITA
More rigid servo holder, more camber link options ... But performance wise I would say you will be equally fast with both.