Thinner straps cut into your shoulders more, which is a bit of a problem already. Also, thinner straps would require a sharper curve into the main body, which would exacerbate the problem of, er, spillage ... for those of us who have a lot of flesh that needs compressing. But then, it all depends on your personal body type whether that would be a problem or not.
As for white vs. black - I agree with Josh.
However, I can tell you from experience that the white ones get dingy pretty quickly. They can also pick up stains from the dye in your clothes if you wear it while sweating a lot. I ended up with blue stripes around the waist line after wearing a new pair of jeans to my very active job.
Whenever I have to replace mine I'll probably get a couple black for every day wear, and one white for those odd occasions when I wear a light-colored shirt.