I have a 1960 diesel. They are foreign to most people because cars don't use diesels. But outside of the injection pump (which I haven't had to get into yet), they are no more complicated than the gas.
I have 14 tractors. The D17 is my favorite tractor (though my D14 gas is a close second). I would not trade my D17 diesel for anything. It is a solid tractor that sounds like raw untamed muscle. And the power is better than any D17 gas I've seen. In low gear, you can't stop it!
Buy the diesel if they are equal in quality. $3500 is not too much for either but you can find them cheaper from time to time. I think the diesel is worth more than the gas.