The manual should fundamentally out-perform the auto in gas mileage. But are you getting older? If you have any foot/ankle issues on your left side then you might want to consider an automatic. If you want to shift then put in a manual valve body in the auto as an option. If you really want to get tricky consider a built TH350 with a Gear Vendors over-drive unit that will give you a six speed auto.

Kitz