Depends on if you wanna go fast or not later.
7.3 will have less issues. Pretty much only see oil leaks, and valve cover gaskets and harnesses. Either one will have front end issues, 05-07 6.0 will ride the best. The 7.3 wont get you anywhere in a hurry unless you have fairly deep pockets.
6.0 motors have more issues, egr, oil coolers, head gaskets, lifters, and oil leaks. Tune only will be faster than a smallis set of injectors and a turbo upgrade on a 7.3, and 05-07 are gonna ride the best and turn the best due to the coil spring front end.
250 vs 350 are gonna be close to the same, dont let that be a deciding factor, unles you want a dually, then its 350 only unless you find a 450 that had a bed grafted onto it. But those ride like ****.