Limit
Pain arc SM Naruto hold SM for 5minutes only
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And it takes time for him to gather natural energy
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So at that period in time, he couldn't exactly fight in SM unless he got the necessarly prep time to do so
It's for that reason Naruto judged it was better for him to train on how long he can maintain the mode to have a chance against Sasuke [
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The Sasuke against Danzo could outlast the 10minutes Izanagi of Danzo during their fight(each Izanagi eye last 1 minute), so in terms of how long they can use their cards, MS Sasuke has the edge over him
Besides, Pain arc SM Naruto could only throw 2 RS in that mode
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And he would run out right away if he does
And i don't think FRS will work on Susanoo either way, so MS Sasuke would edge out U_U