I guess I would say if you are going to spec 50 in weapons, flex is the way to go because of Leviathan. If you were going to go high shield, High SR, then you could go slash but from what i gather, most go crush.
I would like to mention that in thid the slash line is a wee bit op'd, but I'd def go the way of the slam/levi reaver if you're a level 50. I'll presume you all know what slam style is.