A 6.2L F250 should have no problem towing a 7,500 lbs trailer.
As far as gas versus diesel, I think it depends on how much towing you're going to be doing. If it's like my typical experience (8 ~ 10 times a year with trips that are under a 100 miles), with most of the other times you're driving it's everyday commuting you're not getting the benefit of diesel power.
The diesel option is about $10K so if you get 10 more miles per gallon and gas/diesel fuel is $3/gallon it's going to take you over 150K to cover the initial cost of the diesel over the gas truck via better gas mileage. If you have a flex fuel gasser, you're saving close to $0.95 a gallon (my area) in fuel cost so the better gas mileage of a diesel is somewhat negated by the lower cost of flex fuel even though flex fuel isn't as energy efficient as regular gas.
On the highway I'm getting around 16 ~ 18 mpg with the 6.2L 3.73 and while towing I'm getting 10 ~ 11 mpg.