this thread reminds me of Kolox T_T
anyway as for the winner. I wanted to say Chuck Norris but I might as well pick one from the two: Shirou.
Nasu said (ignoring the point of self-destruction for both of them) Shiki would win - this is, most likely, against a Shirō who didn't make a contract with the Planet in order to gain power. Additionally, we do not know if this refers to them at the moment the games they appeared in took place, or if he was referring to both of them at their peak.
t-moon complex x answers the OP questions
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shiki is pure ownage, and shirou is the same retard as he were in fate and ubw
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^It does not answer anything because Shiki cannot kill Servants (once again, that was said by Nasu).
All in all, Shiki is more talented, has uber skill and twice more boring than Shirou. I like Shirou more, but Shiki is strong.
Well, despite Shirou self-sacrificying self-destructive personality, he will outlive Shiki ..ok, not funny that the other one won't live more than his twenties, but somehow I can't feel bad for Shiki because.. I don't feel anything towards that guy. Shirou is kinda stupid and naive but he is a good guy with actually..quite understandable principals. Just because they don't really work for other people doesn't mean he should just abandon them.
But like all V.S. threads, this is pointless.
Aozaki said to Shiki at the end of Tsukihime that he could have dropped dead even tomorrow, so the option of living till his thirties is a very optimistic one.