Greasables suck. They are weaker for one thing, and dunno about you, but I hate going under the truck and greasing 18 different zerks.. it makes a big mess too when the grease is flung out of the joint all over the under of the truck.
I made that mistake with my Jeep. When I change the U Joint I'm going to put non-greasables in there. A few of the U Joints I have to even drop the driveshaft to get to it so I usually don't bother.