Obviously do whatever is right for you, but I've gotta say that I kinda regret not getting MR.
I really do like my MW but if I were to do it again I would get MR, in person the color is just fantastic.
Either way I would get the new one for $3k more. Dealer demos are not treated the best.
2011 E92 M3, 2009 Jaguar XJ8, 2008 S2000 CR, 2008 Evo X MR