I went through the same thought myself, 6.7 or 7.3, and I ended up with the 7.3. I will be towing a handful of time a year around 6k-8k lbs or so, plus I don't put on a ton of miles, under 10K miles/year. For me I felt the 7.3 would do all that I needed out of the truck and just didn't want to spend the upfront cost to getting a diesel. Now, I own and operate a number of heavy equipment that range from tier 3 to FT4 engines so I know understand the positives and negatives of diesel engines. Yes the 6.7 has more power, can get better mileage, and will provide a higher trade in value over the gas engine but you are paying for all that. For the extra $8k you should get more power, better mile, and higher residual value, I mean who would pay that premium if it was only marginally better or the same?
Now longevity is the question, diesel has had a reputation of lasting longer, however I am not so sure how that will hold up with these new engines, plus I have seen V10 fords last for years with little maintenance. Personally I would be cautious to own a used modern diesel engine with over 100,000 miles, mostly due to the new emission systems. In my line of work many guys are moving to lease programs for new equipment due to the cost, it make no sense to buy these new machine out right any longer. So basically you get a 3-4 year lease covered under warranty and then roll over to a new machine at the end. These new engine with emissions can take a lot of money to maintain and keep running long term. If I remember correctly for the cost to replace some components on a diesel, turbo, fuel system, or def system, comes close to the same cost to replace the entire 7.3 engine.
When comparing gas to diesel its not the same comparison it was 15 years ago, and more specifically the 6.7 to the 7.3. The 7.3 actually develops a lot of its torque at low RPM not red line like other gas engines. So in a way the 7.3 is a little like to 6.7 in it develops its power at a lower RPM which helps when towing. Not saying they are alike but just remember the 7.3 is a new engine and not designed like the 6.2. We don't know about the reliability, resale, or longevity of the 7.3, only time will tell, but it does look promising, plus the simplicity of the engine is great.
Tow a lot or put on a ton of miles the diesel makes sense. Tow some, drive some, and plan to own the truck short term you could go either way. Looking to keep the truck long term and don't tow much or heavy I would almost lean towards the 7.3 on this one. In the end it is your time and money so spend it on what makes you happy and works for you. Both are pretty good options just need to decide which one fits your situation best.