I like both so I'd go with
-MC if you treat your items well and figure you'll probably buy another one in a few years or
-damier if you're harder on your stuff and want something bulletproof that you'll be able to use for years and years
That's a great reply from ValleyO ... I suspect you won't get better advice. For most of us, our LVs are expensive purchases and so we want to make good choices, to buy pieces we'll get a lot of use from and that will also last the distance. Problem is some lines, like MC, are not so durable, and some items, like a keyholder, take a lot of wear. They're forever being handled and going in and out of your bag.
I fell in love with framboise vernis and wanted to get matching accessories - wallet, agenda, keyholder. I figured, even with care, a vernis keyholder is going to be showing its age long before the other pieces do. My solution - two keyholders, one wallet, one agenda. (I posted about it on tPF and had the amazing good luck of having two PF members offer to sell me a keyholder ... K and R, thank you!!!)
I guess what it boils down to for you is the damier is the more practical choice, versus the prettier but more delicate MC. It will be really interesting to hear which way you decide!