It would give them more power in the UK than anything that has happened since James was crowned.
I dont see why Sturgeon would risk that. If they join with labour to form a government, they can pull the strings in the background. Ditto with a weak minority government of any persuasion. In both cases they can win ever more concessions for support in enacting any legislation they like. If they let the Conservatives form a government, possibly in league with other small parties, then five years of a Conservative England and an SNP Scotland is going to broaden and deepen the gap between the two, weakening the Union ever more, which of course is their ultimate objective. If affairs pick up in Scotland while they are the majority party, then they will take all the credit, strengthening them even more. If things go bad, they'll just blame the government in Westminster.
I just can't see any scenario where they do not win, unless they make some catastrophic blunder. Joining with the Tories would count.