I can't believe I'm saying this- but I voted putty. It's a gorgeous neutral! I think the plum is too blue for my taste and I wish it were paired with silver hardware . . .
I am like you with the neutrals, although I now have something to work on in my collection which is adding a few bolder bags...having that said, with an investment bag such as this one, go for PUTTY. My philosophy is, invest in the classic bags and pick up the trendier bags at the outlet or on sale....just my 2 cents for what's its worth.
BUT I'm gonna say go for plum! That way, you'll have a burst of color to pull out when you're feeling particularly flirty or need a break from the neutrals. And as trendy as the color may be now, I'd argue it's a color and style that'll last. And it's a fun accent to an outfit. AND if you already have a lot of neutrals, why not get something different?