Either will work fine, I usually go with greaseable though. Non greaseable are stronger if you're going to be hard on them.
That's not always true. Some non-greasable should be stronger because they are solid like Spicer, but I've seen plenty that were hollow just like the greasable ones, the only difference is that the ends weren't drilled out.
The debate for off-road and keeping dirt, mud and water out has raged on forever. The big difference is that if you grease them, you can flush any dirt. If you get any contaminates in a non-greasable joint, you'll never know and there's no way to make sure you didn't.
I think the truth is that if you do regular maintenance on your vehicle, you buy greasable ball joints, outer tie rods and u-joints. If you do nothing more than change the oil occasionally, you're probably better off with non-greasable in everything.
