I just got the 2011 F450 and it's FANTASTIC! I already posted to the other F450 post so check that one out for more. As Shelsay mentioned and I forgot in my other post, it's incredibly quiet. When I picked it up, I was standing about 5 feet away and a 2010 6.4l (which was significantly quieter than the 6.0l) started and idled by about 10 feet on the other side of the 2011 and drowned out the noise from the 2011. Others have mentioned the power, I just took my 8.5X24' cargo trailer on about 700 miles and couldn't feel it behind me which is quite different from the 2008 F250 I used before.
As for the mileage, it's better than my 08 6.4l F250 was after being broken in. If you're towing 18,000 (the limit for the 2011 F350), I'd go up to the 450. It's nice to not be at the limit and the extra pulling power is nice. If you're under the limit, go for the 350, insurance will be easier to obtain and you'll save a little cash both on purchasing and on fuel when on the highway with that lower differential.
I also didn't like the fenders, but I'll get over it. The improvements well outweigh the drawback of ugly fenders.