There's a reason why the 2008-2010 powerstroke is much cheaper than the Duramax and Cummins. That being said, my buddy owns one and it runs great since he did the emissions "fix". If you want a fifth wheel, I would recommend the 350. I am currently running the setup in my signature and I'm 1,000 over my GVW, "fixed" with airbags (I do have higher rates tires than stock and I'm under each axles weight rating). I personally would rather have a 350, but that's not in the cards monetarily right now. My pickup is paid off and I need it to stay that way for a couple more years.
Go to Ford forums and read up on the 6.4 engine. It's a good engine if you know what you are getting into and don't mind tinkering. If you don't want to tinker, spend more money up front and you'll come out ahead.