Obito wins
I'ts been shown multiple times that obito can evade attacks that he's unaware of. [
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] If obito can become intangible even when he's attacked without knowing it, then invisibility won't make a difference.
The only time you can hit obito is if you hit him as he's just about to warp you, & even then you need an instantaneous jutsu so that his intangibility can't react in time, otherwise it overrides everything.
Not to mention obito has izanagi, & he's survived much worse than a sword stab. [
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] So if muu tries to do anything besides decapitate him [which izanagi can be used to recover from] then obito tanks the stab, grabs the sword, & warps it along with muu himself.