King pins last a lot longer, especially with added weight (big motor, plow) or big tires. I think they are less likely to come apart down the road as well if they do go to H$$$.
I replaced my king pins at 350k miles. Truck had a plow it's whole life and was worked hard. Turned out it wasn't even the kingpins that were bad. I was trying to track down death wobble.
I replaced the ball joints in my K5 about 6 times in 200k. No plow, 33" tires, frequent trips through the drink, fields, etc, but no extraordinary abuse.\
I'll take king pins any day over ball joints.