If you mean acoustic guitar, it's a no brainer. The 4040 would be much better on acoustic guitar, and would be usable for vocals. The M9 might be good on your vocals depending on your voice, but would probably suck on acoustic guitar.
Don't get me wrong, I love my M9---it's a good mic for my voice---but if you're trying to use one mic for two purposes, you need a much more general mic.