Should I use to file my spouse's 1099 on Sch 1040C or on SCorp Sch 1120 for our newly formed single member LLC?

The default tax treatment of a single-member LLC is a sole proprietorship. To be treated as an S corporation, you would file Form 2553 to make the election. If you didn't make an election for the LLC to be treated as an S corporation, you would need to file income and expenses on Schedule C.

You have 75 days from the date the LLC is formed to make the election to be treated as an S corp from the beginning. If you haven't made the election and you intended for the LLC to be treated as an S corp, you might be able to file a late election if you have reasonable cause. If you want to to do this, you'd need to file the 1120S this year instead of the Schedule C.

Click here for more information about S corporations.