>>6130000I think K2 looks way better than the Black Series; the sculpting is sharper, the proportions seem far more accurate to me, and the SHF has a great sort of dull-gloss finish that looks perfect for K2. Articulation is also excellent with strong, tight joints where you'd expect them and good range of motion. Holds poses and balances very well as he's not particularly heavy.
On the downside, he lacks any sort of weathering, which was probably the main thing that visually set K2 apart from the other Imp security droids in the film.
The better proportions of the SHF were more important to me than the weathering, so that's what I went for, but if that doesn't really matter to you the Black Series is probably all you need.
Same kinda deal with the Deathtrooper. I think the Black Series has overly long limbs, whereas the SHF looks much better to me in that department. SHF articulation is also better, making varied firing poses easier to achieve. Still, the Black Series is a good (and good-looking!) figure for the price, and again if you think the Black Series looks good it'll likely serve you well.
In short, neither K2 or the Deathtrooper are really stand-out releases. I think they're better than their Black Series counterparts, but not so much better that I would unequivocally recommend them over the BS (whereas I would absolutely recommend avoiding the BS First Order trooper in favor of the SHF).
As for the Specialist Deathtrooper, he's apparently going to be an exclusive release with a preorder/release date TBA.