grammaticality - Should I use "important" or "importantly" in the introductory phase of a sentence?

Should I use important or importantly in the introductory clause of the second sentence:

Appellant's argument is premature. More important, it lacks merit.

Google NGram shows a significant preference for "more important." Recently, however, it appears the democracy of usage has swung slightly in favor of "more importantly."

I prefer the former, but I am more concerned about learning which one is grammatically proper, and the particular rules that control the analysis. Can someone walk me through it?