I think it comes down to whether you're a Dodge or Chevy fan.
I would consider the Chevy if the condition is great. Love my 8.1. I get 9.5mpg highway towing my 7000# 34' TT with the 4L85E 4speed tranny and 3.73 rear. I don't daily drive it. Unloaded it averages 12mpg. The 5speed Allison is rated for higher torque(620 to the E85's 460ft-lbs) and has a hair lower overdrive(.71 to the E85's .75) for possibly maybe better mpg's. The 8.1 can use a quart of oil every 2 to 3k. By all accounts this is normal. I just see it as "You gotta feed the beast".