You've already got the automatic so that will be the cheaper way to go. The OD setup will most likely get better mileage but you'll spend at least an extra $500 rounding up the bell housing, flywheel, clutcth & linkage, etc. to make it work if you have to piece everything together. If you could find a car with all of the clutch & bell housing stuff in a pick-a-part type salvage yard you might be able to get everything for maybe $125 or less. Then you'd just need a new clutch.
If it was me, I'd prefer the OD setup because I like to shift gears.