It depends a lot on the year. If it is a 2015 then there is little difference other than the Sierra 2500 has an independent front suspension and a smoother ride and shorter turning radius.
The Sierra (starting in 2011) has a much stiffer frame and is more on par with the Ram 2500 trucks from 2014 to 2015. That is most noticeable with the DRW trucks where the wheels are further out from the frame and exert more leverage and twist the frame more as a result.
Ford has more options available which is both good and bad. Good in terms of options but bad in that it can be very confusing as to what you get exactly with each one and this is where I would not for a second trust the opinion of the dealer. For example there is a heavy load package that increases payload capacity and a towing upgrade and it is not clear which one provides what and with a 5th wheel will one want to have both.
I would compare fuel tank capacity and the availability of a Titan tank for both trucks. Great to have an extra 20 gallons and 20 miles more range when towing.
As mentioned the comfort of the ride and the handling is also a consideration.
As for towing there are useful articles with actual towing tests done by etrailer.com and Car and Driver of the different 1-tons and the results are very much applicable to the 2500 trucks with the diesel engine. This is where the ability to pull up a grade or accelerate to freeway speed with a heavy trailer is more likely to be noticeable as is the grade control and exhaust brake function with the different trucks.
With the 2015 I would be looking at Ram as well. Ram sells more 1-ton trucks than either Ford or GM, has the strongest frame, Cummins engine which uses some parts in common with heavy duty truck engines, and has an excellent exhaust brake.
I have a 2011 GM Duramax and I was surprised at how perfectly the grade control feature works with the tow/haul mode and with or without the exhaust brake engaged (which can be engaged or disengaged at any time with the push of a button on the dash). I have come down 8% grades with a heavy load in tow and never had to touch the brakes and the truck stayed at 35 MPH. A tap on the brakes and the truck would automatically downshift and maintain a 25 MPH speed to get through a section with tighter curves. The downhill control is where I most appreciate having the very expensive diesel engine under the hood.