Welcome to our M5 site!
I'm fully with you on your thoughts of getting an exclusive M5 color (it's what I've done in the past as well with the M3), but something to keep in mind is that these two exclusive colors may also end up being two of the most colors for the F10 M5.
MCB is the marketing color so as you said, it's already popular. And judging by the very positive reaction to photos of singapore grey M5s posted here, I would assume that will be popular as well.
If you're leaning towards singapore grey I don't think you can do wrong. I haven't seen it in person but it seems like a very dynamic color for a darker color. To me, it has this great slight bluish hue to it that makes it avoid having the dullness of a traditional dark grey color. Don't know if you've seen all the photos posted of this color, but as Southlight suggested, have a look at our
Singapore Grey F10 M5 thread (your pics are now included as well). Maybe there are some photos there that help your decision.
But if you're looking for flashy, MCB might be the way to go. Sure it might be popular on dealers order lists, but how many will you bump into on the roads. Only on the internet will that color feel 'crowded'
Hope that helps. Be sure to share your decision with us and of course your ownership pics and experience!