I have driven a Kenworth w900 with a Cat 425, 13 speed and now drive a Mack 427 with an autoshift. Every day I want my Kenworth back; everyday I want my 13 speed back; as for as motors go, it depends on so many factors such as rear end and transmission. To answer your question: if you want to control how you run your truck get the KW because the automatic is all about fuel savings and low reving. GOOD LUCK