I say forget the bean counting BS.
In this size of a machine, diesel is the best option. For a garden tractor these are heavy machines. Gas powered X7's drink fuel like a fish drinks water. While opting for a diesel version will increase your payment only slightly ($20 a month probably) you will notice the difference immediately. Resale value on a diesel is higher, regardless of whether you keep the machine short term or long term, the diesel WILL pay for itself.
Add EVERYTHING into the equation. Those who have calculated the fuel cost savings forget some important details. The more often you have to get fuel, gas, in this case, your spending money to get it. You have to get into your personal vehicle, drive to the gas station, buy the fuel, and return home. With a gas tractor, these trips will be VERY FREQUENT. Now, with a diesel X7, not only are you saving money on operating costs, but you will make very few trips to get diesel fuel. Saving wear and tear on your personal vehicle, saving you fuel in that vehicle, and giving you more time to accomplish useful things instead of making trips to the gas station.
Gas engines require more maintenance, such as spark plugs, on a regular basis. The diesel Yanmar engines are almost maintenance free besides oil changes and fuel filter changes. Besides that, you turn the key, and you go to work.
Once you go over the 20HP mark, and you get into the 1,000 + LB range, diesel is the only way to go. Gas engines will bog down in heavy thick grass. Diesel engines say "what's that?...did you feel something?...must have been a mosquito"
Please, do yourself a favor, and get a diesel engine. In this day and age, EVERYTHING should be diesel. They are extremely efficient, last forever, have gobs of power, and sip fuel.