The Allison is the better transmission. It's a 5 speed. And it is more of a heavy duty trans. My buddy has an 8.1 allison extra cab non dually he gets 12 miles per gallan non towing and he pulls a 32 foot travel trailer and he gets 9 with it. He's got 3.73 gears. I know your looking used but I would get the allison if I were you. But if the price is right the heavy duty 4 speed should be OK. But with your boat I would want a 4.10 rear. Give me a shout and I'll go shopping with you. 2 heads are better than 1 when it comes to looking at used trucks. How was that 96 I told you about?