I've had both with a 6k trailer (several 1500 hemi's and several 2500 hemi's)...
My 6k trailer has ~900 lb tongue - I'm over GVWR on the 1500's with passengers/cargo/wood... Extra payload of the 2500 is nice ;)
The 1500 will get 16-18mpg EMPTY on the hwy, 2500 will be in 14-16mpg EMPTY
Loaded they will both be very close to 8-9 MPG..
Certainly pros/cons to both which is why I've switched a few times.. Personally I would stick with the 2500.. Both have similar power but the heavier frame/brakes/axles will 'handle' that size trailer better.
Just my $0.02.. :)
