I'm not a huge fan of LACs myself, so I don't prefer the 6D. That said, a big part of the question is what sort of terrain you're in, and what sort of opposition are you expecting. I think with a bit more range, the 6X is more of an all rounder. More range tends to mean more damage, even if it's generalized damage, and in a team setting the clusters can often generate the same crits as AP rounds. Now, it can't hurt infantry the same way, and it can't load precision ammo to hunt lights, so it can't specialize the way the 6D can (and with 6 tons you can take on two or three specialties). So if you know or even expect to face foes that the 6D can specialize against, you can get your money's worth. But, for me, if I don't have an expectation (and some of this goes to my usual player opponents and what I tend to see from them) then my baseline is to expect mostly mechs as a default and so I'd default to the 6X.
That said, I'm a big fan of the one with the LGRs and the big suite of ER MLs, though I'm a big LGR fan.