I like the MAFEX Trooper fine, I prefer the BS6 by a slight degree, but if the Hasbro one had looked like that I would have been happy with it. The size difference is enough to make it look like it's from a separate line, so I wouldn't try to mix the two together. But the biggest red light for me is the price - I can't imagine how anyone could argue that they are enough better than the BS6 Troopers to justify the huge price difference.
As for the Vader, there are things I like and things I don't - much like the BS6 Vader. Without actually holding them in my hands, I'd say I have a slight preference for the MAFEX Vader over the BS6. I'll be interested to see some side-by-side shots to see if I could get away with mixing it in with my BS6 figures. If so, I might do it as long as the price doesn't go over $35 - and even then, the in-hand photos would have to look really good.