I spent a lot of time debating between a WWM and a WW (both single layer). In the end I went with a WW for a few of reasons:
1. I liked that the WW came in silver. I've really liked the color since they introduced it.
2. The WW was a little wider than the WWM.
3. I'm not a gram weenie (yet) so I didn't mind the extra 0.5-1 ounces that the WW weighs over the WWM.
4. I wanted the slightly heavier fabric from the WW--see number 3.
Hope that helps.