Right... Of course Mewtwo itself loses against Sigilyph on its own. If they were the only two things on the field, Sigilyph would win; that is obvious. Mewtwo can't damage Sigilyph, so of course Sigilyph wins that irrelevant, hypothetical situation. We are trying to get actual discussion out of this thread for Sigilyph's use in the real metagame, not match ups against deck concepts that don't exist. Sigilyph would beat Mewtwo by itself on any day of the week, unfortunately, we live in the real world where any deck that is carrying Mewtwo EX will also have another attacker that can effortlessly handle Sigilyph's 90 HP. This makes Sigilyph a bad use of a card slot in my opinion. I would much rather direct focus towards deck consistency, or a tech that is less easily played around, like Lost Remover.
As for Sigilyph being better than Mewtwo alone... Just a straight forward NO there. Sigilyph is good against Mewtwo, but bad against everything else, while Mewtwo has a practical use against almost every deck in the metagame. This is probably why it is worth 60 + dollars, and everyone who owns it plays at least 2 copies. Please, don't bring the point of Sigilyph beating straight Mewtwo EX up again, because that is not a played deck, is obvious, and is a distraction from discussion about Sigilyph's applications in the real metagame. Sorry if I am coming off as a little rude here, just trying to get my point across in the most clear cut manner I can. I'm not going to tip-toe around my points right now.