8 3/4 vs 8 1/4 weight

If you don't need the increased strength and stopping power, why bother with the changeover?
If you DO need the strength increase, then you NEED the strength increase and an extra 100 lbs (and it won't be anywhere near 100, might be 50, I would bet less) won't matter.
You haven't told us why you think you might need an (expensive!) 8 3/4.
Edit: lightweight (drag racing) disc setups (without parking brakes) are a bit lighter than small drums setups, more lighter than the 11"drums. Street disc conversions for the rear with parking brakes are quite a bit heavier than 10" drums.
2nd edit: if you are after light with strength..... 8 3/4 with 9" drums. You have disc's in the front that do 80% of the stopping, right?