In Birkenstock a "Narrow" width is more like a women's medium. A "Regular" width is like a women's wide width. They use Euro Sizing. For example a 39 Reg would be like a women's 8-8.5 wide. To figure out your size, add 31 to your shoe size and that will most likely be the size you wear. (31+8=39)
The Soft Footbed has a layer of memory foam put over their classic footbed. The support is still the same great support, it just doesn't feel as "hard" at first. The soft footbed is great for a first time Birkenstock wearer - easier to break in.