First go with the Birkenstock brand.  Just got last night.  Wearing in the house with socks since it is 6 degrees here now.  A little wider than I expected but glad I didn't go narrow.  Have high hopes.  I just today received tthe  Gizeh(spelling?) Thong.  It feels like it was made for me.  HIgher hopes for the thong.  I had read that the women's styles are a little narrower than the unisex which stands to reason and appears to be true.  I wear an 8 1/2 or a large 8 and the 39 medium was the perfect choice but a tad large.  Gave 4 stars as I don't know enough yet.
***UPDATE****  I was WRONG about the size.  Go smaller and don't make my mistake.  I am now stuck with 3 pairs of Birkenstock in the wrong size because I didn't due enough homework and have been wearing with the socks.  But........  I love them and will try to get my daughter to wear them since they fit her larger foot.  By the way, I have ordered 4 pair of different styles in my actual size as I have seen wonderful and positive improvements to my legs, hips, and back just from wearing a few hours a day.