[wingnut60]

Rick,
I had a 2004 350 SRW that I abused for over 120k miles, when the oil cooler blew and let oil into the cooling system. Fixed for $2100, then traded for a 2007 DRW that got me to 95,400 miles with only an single injector problem (warranty). At that point, having towed the MS a lot and noticing small things that indicated I might have problems soon, traded for an '09 450.
Towing a heavy trailer for a lot of miles is straining a 350, but it seems that it is up to the job. I opted for the 450 to get all tow ratings correct with the idea of keeping it a long time. Now at 55k and only had a couple oil leaks fixed under warranty.
If you are not having any indication of upcoming problems, you might consider checking into a Ford DieselCare engine warranty--I believe you are still eligible--that would help after 100k on the engine.
My 6.4 has done everything I ask of it, but fuel mileage is between 7-8 when towing, and 13 when not. The '11 450 is mainly a 350 with higher tow ratings, and the 6.7 supposedly gets better fuel mileage.
One of the benefits of a 450 is the shortened turn radius--until you have driven one, you won't realize how great that is over the 350.
Joe