I have no experience with a 9 1/4 so I can't comment on their strength. If your car is light weight, auto and doesn't hit the tires hard the 8 3/4 may be fine. Mine broke on the second pass after installing the 408 in my 3260# Dart. It couldn't take the 1.38 60'. After removing and disassembling the 727 to check the sprag I rebuilt the 8 3/4 and promptly sold the complete rear end. My Dart now has a Strange S-60 in it.
You can find 9" parts pretty cheap and IMO it's a better option than either rear you have for a car with decent HP.
As far as parasitic drag goes I don't think anyone on this board with a street or street/strip car would notice it.