You need to decide on what disk brakes you are going to use before you worry about which ball joint to buy. per 73 factory disk brakes will force you to stay with the small ball joint, the 73+ will force you to go with the big and aftermarket brakes can be had for either.
The 73+ ball joint is bigger and stonger and is also cheaper than the earlier A-body joint but it also requires different spindles and upper control arms.
There is no inherent weakness in the pre 73 A-body ball joint it was just an effort on the part of mopar to consolidate parts.