The only problem with a Cummins is that they come wrapped in a Dodge. Paint issues, crappy interior pieces, weak automatic trans, etc.
Stay away from the non-turbo 7.3s unless you just like going slow. The turbo models respond well to tuning and can make gobs of power, though that necessitates a built transmission,
The 6.0 Ford can be a good engine as they had the bugs pretty much worked out by 2007, the last year. Mine has been great but there are some upgrades ($$$) that can be added to make them bullet proof. The 2005-07 Fords have a higher payload then later trucks (trucks got heavier in 2008) and sit a bit higher for off-road clearance. My 2007 F350SRW has the same GVWR as an 07 GM Dually. No need for smog, DEF, Catalytic Convertor, or ULSD with a 6.0.
GM trucks sit lower (even 4x4s), ride nicer and carry less weight then the corresponding year Ford/Dodge trucks.