In a nut shell diesels are superior at towing. However, gas engines have improved at leaps and bounds over the last few years. Another very strong point is the transmission in today's tow vehicle's which will make either engine shine!
You have to decide for yourself which choice is best for you. But I would not hesitate to buy either of the big 3's gas or diesel engines of today. I personally have a 2012 F-250 with the 6.2 and it does fine. A few things you should consider such as I did are......
1. Intended use for vehicle "most" of the time. Daily driver? Dedicated tow vehicle? Or combo of both?
2. Payload and rated towing capacity. What are you gonna tow? What's it weight? What's the pin weight/ tongue weight?
3. Cost? Purchase price between the two? Fuel? Maintenance? Future resale value?
4. What you like!
#4 was a biggie for me. I liked different things about all the big 3 manufacturers. But in the end I am just a Ford guy and liked there trucks the best. And the 6.2 has a very, very good track record so far and is proving to be a very reliable and tough engine. Fuel mileage could be better, but its a 3\4 ton truck and was purchased for a specific job and need so I accept it.
Its a mind boggling process in choosing and I sympathize with you. I spent months before I made a decision and its a tough one.
Good luck!