if you tow more often than not, go 2500. I had a hemi 1500. My travel trailer is about 8000 lbs. My work trailer is a bit more. a year into ownership, I made the switch to a 2500 with a 6.4 hemi. Lost a bunch of money but lesson learned.
the 1500 pulled the trailers OK but everything else felt a bit too much 'white knuckle'. when the trailer weighs more than the tow vehicle, it really effects the dynamics of turning, stopping, swaying…
towing, gas mileage is a moot point, they both suck. as a daily driver, the 1500 really is better on gas. probably about 5-6 mpg better with my city/highway driving. having said that, you only need to poop your pants once and 5-6 mpg seems a small price to pay.