The question of which version is such a tough one.
Personally, I think an Ultimate Vader that can encompass every appearance is the way to go.
Will it cost a lot more than just a single movie representation, absolutely. Will a lot of collectors be frustrated spending money on extras they don't care about, totally.
However, if they do individual figures, there's potential desired incarnations won't happen.
I want a ROTJ reveal figure and an ANH Vader to go with Old Ben, after that, I may buy ROTS and ESB, but I don't feel a need for them.
Now suppose they do ROTS first. It'll have decent sales because it's the first SSC Vader and people are chomping for one. So Sideshow decides after that, let's do an ESB Vader. They do, but since many already bought the ROTS, and it probably cost them a bit, sales aren't too strong for ESB, so then it looks like it may not be worth making ANH or ROTJ Vader and many who wanted those never see the Vader they hoped for get made.
I'm sure Sideshow will make the best choices in the end, but it certainly isn't an easy choice to make and I'm sure there's been many a meeting to come up with the best approach.
Personally, I think at most, the line needs 2 Vader figures to feel complete.
We need ANH as it embodies the first visual of the Dark Lord we ever have and compliments Old Ben as a symbol of the powerful dynamic between Obi-Wan and Anakin, and we need ROTJ with a reveal feature because of the power of those brief minutes when after 2 films of seeing this evil soul, we get the redemption of a good man and to me, one of the most emotional moments in the saga, to see Anakin dying and Luke finally having the father he'd always missed having for a brief moment, great stuff.