I own 2 MB bags and have fondled various WM bags. I probably would have bought the WM Summerlite but hated its hideous red color (yes, subjective, of course). In any case, my MB UL down No. 3 is also a wonderful bag that I find true to its temp rating.
For me, both brands are winners. I wouldn't fret over which brand to buy -- it's hard to lose, either way.