I don't know if it's true or not but I've been told by unofficial enthusiasts that the best to stick with is the 3500 because of with the 45 an 55 fca knows your going to haul big, that's why you buy those models, so there tuned down. Even though they run the same motor an tranny, I've heard they tune then down because there mostly bought for commercial hauler use an don't want them Getting over worked . This maybe all bogus BS, I'm sure we'll get lots of opinions on this one, so come on fellas.