I *sort of* see what you mean, but I think if you watched 'Sith' first, then the shock of 'Empire' would be gone, whereas if you watched them the other way around, you'd get both the shocking revelation of 'Empire' AND the tragedy of 'Sith'. Tragedy works on the notion of the hero, fatally flawed, fighting against and eventually coming to grips with his destiny/fate. Pity and catharsis is evoked in the audience as we watch the hero fall helplessly to the doom of his own construction.