If the 912 chassis is in good shape it would make the "better" conversion from many factors: chassis simplicity, weight, aero (top on), etc. Since you can buy an EV these days for a lot less than it takes to build an equivalent performance one (especially if you buy one a couple years old) The value proposition for the car has to be in the uniqueness or novelty of it, and a vintage porsche should beat a 90's BMW any day on that count.
That said, ultimately it comes down to which car you most want to spend $10K or more on and want to keep as long as possible and drive as much as possible, but knowing its resale value won't be much. If you do convert the 912 consider doing it in a 'reversible' way (don't cut the chassis to make room for batteries) That will help preserve value as a vintage car. With the BMW, hack away, who cares...
Good luck
That said, ultimately it comes down to which car you most want to spend $10K or more on and want to keep as long as possible and drive as much as possible, but knowing its resale value won't be much. If you do convert the 912 consider doing it in a 'reversible' way (don't cut the chassis to make room for batteries) That will help preserve value as a vintage car. With the BMW, hack away, who cares...
Good luck
