just ordered my frt disc brakes

The lower ball joints carry the weight of the car. The upper ball joints just stabilize the top of the spindle. Most of the loading it sees is side loading due to braking and steering.

Having said that, the purpose of putting discs on the car is to improve braking performance. Well to do that, something has to absorb the extra energy. And think about this, the front brakes contibute 80-90% of the braking power because of weight transfer. I don't pretend to know exactly how much of that force is put on the upper ball joint, but it HAS to be some.

Another thing to consider is tires. Those drum brakes and spindles were designed for little skinny bias ply tires. I am running 215/60 radial tires on the front of my Dart. That is considerably more tire (larger contact patch, stickier compound, more weight, etc) than what the suspension and brakes were designed for, and I am sure it increases the loading forces tremendously on the ball joints and suspension bushings.

So it's like Hawaiiduster says, if it's a strip car, or limited weekend cruiser, then the smaller ball joint are probably fine. But for a car that will see agressive street driving or autocrossing, definately go with the biggest and best you can get.