If its either/or thats a tough decision. "Hair down" is more important in terms of the film but the regular 'assassin' version is a really cool look and has a superior likeness. I wouldn't want to be without either but if I was choosing I'd also consider that the T-1000 as Sarah figure has more compatability with other T2 figures like the T-1000 himself and the BD T-800. It has everything the assassin Sarah has apart from the machine gun, hat and sunglasses. So its otherwise the same body, clothing and weapons including the shotgun, pistol and knife. You would also have the steel mill floor base and T-1000 bullet hits which if you wanted you could add to the real T-1000....oh and the donut head!
So ultimately I think T-1000 as Sarah is more worthwhile but anyone requiring only a standalone representation of the character should get the regular figure for its better likeness and all around coolness.