If your reason for tradin' up is to not be loud rattly and rough then the GMC wins hands down.
Lotsa variables that you have to consider, condition of both trucks, major repairs done or will need to be done on both trucks. History on the 03?
I've owned both and the plusses for the Dodge are looks, can get a 6speed (GM too in the LB7 but not as common), Cummins runs forever.
Downsides of the Dodge, rides rougher, 2nd Gen trucks just drive sloppier in general than the GMT800? trucks, VP44, plus what you said.
Upsides of the LB7, quieter if that's what you want, Allison trans no explanantion needed, again very reliable engine save for some injector problems (Who puts 500kmi on their truck anyway? If it blows up at 300-400k I don't give a chit since I won't own it then.), more refined truck in about all respects.
Downsides to the LB7, injectors (but I know myslef and others that had NO injector issues), ride height (if that's important). Personally I thought my 02 Dmax was a better truck than 2nd Gen Dodge. That said I sold the DMax with NO issues for a 3rd Gen diesel Mega Cab.
Apples to apples an 03 Dmax ext cab is not worth $9k more than an 01 Cummins with the miles you're talking about, but............
depending on maint, condition, mods, accessories, etc it could be worth $9k more.
