The new 6.6L chevy gasser or the 7.3L ford gasser are both potent towers. I would say anything up to 16,000 lbs and 3500-4000 hitch weight is OK to tow with these trucks. The new diesels are complex beasts that require crazy maintenance and are prone to pump, turbo, emissions etc. failures where you will either have to maintain an expensive warranty (extended) or eat up the cost - a CP4 failure on the new powerstrokes can easily cost you up to 12K to repair and ford has frequently denied these claims blaming bad fuel for the issues (it is the Bosch CP4 pump that has a problem so....).
An F-350 SRW gasser or a 3500 Chevy SRW gasser should have payloads in the 3700-4000 lbs and plenty of towing capacity.
At current gas and diesel prices assuming you tow about a 1,000 miles per month, you are looking at 80,000 miles to make up the $10K premium you pay for a diesel engine. Add to that the fact that most powerstroke people tell you to do oil changes every 5K miles ($150 at the dealer) and oil+fuel filters every 10K miles ($350 at the dealer), you are looking at extra $4,000 in maintenance over the course of the above-mentioned 80,000 miles. Did I mention lubrication additives with each diesel fuel-up? Boy, you think you are pampering a machine made out of gold! Anyway, this realistically means you will take 100,000 miles or 10 years to make up the diesel engine premium! And there are no risks of CP4, turbo, emissions etc. failures, no mystery coolant leaks...
IMHO, only people towing > 16,000 lbs and people who work out of a truck (tow things daily like tractors, heavy equipment etc. to work sites) need a diesel. The whole myth of the "reliable and runs forever diesel" was built on the pre-2004 7.3L powerstroke and pre 2004 12/24v 5.9L Cummins engines - they were easy and cheap to maintain (but then they had the same torque output as the new 7.3L ford and 6.6L chevy gassers so you weren't getting anywhere in a hurry). Today's diesel engines put out 450+ HP and 1000 ft/lbs torque and are SUPER complicated. Just open the 6.7L engine bay and then open the 7.3L gasser engine bay. You could drop a screw driver in the former and never find it again...