Before I state my biased opinion, I want to point something out.
When you say "more power" that can be misleading when comparing a diesel to a gas engine. A diesel typically makes more torque at lower RPM than a roughly equivalent gas engine. Torque is a MEASURED value. Horsepower is not measured, it is calculated. If an engine makes the same amount of torque at a higher RPM, it increases the resulting calculated horsepower figure.
SO, comparing these engines based solely on the horsepower rating can be misleading, and can be an oranges to apples comparison.
Now to my biased opinion. I have low time 725K2, about a 2005 model. I LOVE the machine. It is great in every way EXCEPT it has no power deck lift and is an air cooled Kohler. Nothing at all wrong with the air cooled Command Pro Kohler, it's just that it's not a diesel. I would rather have a diesel. Buying a used machine you get what you find in good condition. Buying new, you have to ask yourself if extra money for a diesel is worth it. For me, if I were buying new and could it afford it, this would be an easy decision. DIESEL!
My humble opinion. Hope it helps.