All 3 bags are nice, White Mahala gets my vote. =) I personally don't like the color combination of the Zac Posen bag, plus, I find Muse looking more gorgeous in other colors (Chocolate, Ivory, Violet).
I am from Southern California, I wear my white bags year-round. How to keep them clean? Clean them as often as needed, don't let stains/dirt set in.
Good luck with your decision & keep us updated. =)