Re: re-gear or re-prop???
Several posters have waltzed around the answer, but nobody actually said it.
It is kinda like whether to change the transmission gearing in a car or change the rear end gear (final drive) ratio. As long as whatever you change results in the same overall system change, it won't matter where you make the change, the performance will be the same.
For all practical purposes in a boat it does not matter whether you change the stern drive gearing or change the size and pitch of the prop, as long as the end result (the final drive ratio for lack of a better term) is the same, your engine and your boat performance won't know the difference.
Obviously it will be much easier and it will cost much less to change the prop, if you can find the right one.