Great input, all! Sveta and bj1111, I took a M in this jacket. I seem to hover between S and M in their jackets -- the split-sleeve blazer I bought is a S. By way of comparison, I'm typically a 6 or 8 in jackets and my Helmut Lang jacket is a M.
The fact that the jacket is not terribly heavy or warm is a plus for me -- I'm finding I run warmer these days and heavy jackets are not so appealing to me unless its the dead of winter. It's too hot here to wear jackets in the summer, so this would be a fall-winter-spring item for me, like the black one is. It's actually not hard to layer over this -- I've worn my Burberry trench over it, which is not ideal (the jacket hem peeks out from beneath the trench), but it works. In colder weather, my burgundy wool coat fits over it.
The white is definitely making me hesitate, mostly for practicality reasons. It's a long jacket and would potentially drag on various surfaces when I sit down, etc.
I'll think about it. I suspect a long structured jacket in a different color and maybe a slightly different cut would appeal to me more, but that's a HEWI for sure. Especially when you consider that it's often challenging to get a good fit on my hips as well as my upper body, and accommodate my short-waistedness! Sometimes I wish I could sew.