Not sure, but here are the pluses for the 9\"
I know a lot of drag racers run the ford 9". There has to be a reason! A LOT of heavy duty parts are available for it, and it is easy to swap gears in. Lockers and limited slips are plentifull also. It can be narrowed easily because of NON-tapered axleshafts In case your putting it in a Jeep, early Bronco or BroncoII I think the houshing is small compared to most, so it may provide better ground clearance. I'm not very familiar with the 12 bolt, but I think my dads truck has one, and the housing sits really low. Is the pinion raised above the centerline of the axles on it? I can't remember. Anyway, What kind of Truck are you putting it in? you may have to fabricate axle and shock mounts too. Just my input, maybe someone else will know more about the 12 bolt.