It depends on A) your budget, and B) your portability needs.
If you can afford the MB and don't absolutely require the portability of the Wind, then I would really recommend the MB. Even if you only want to do relatively light work, it is still better done on a MB. The Wind actually runs Office, web, stuff like that pretty well, but the screen size and small keyboard are limiting (obviously a tradeoff for portability.) The Wind even runs OS X very well if you decide to go the hackintosh route.
OTOH, if you want to save a few bucks and need something really light for occasional use, the Wind is a nice little netbook. Keep in mind the 3-cell battery is really weak, I would recommend buying one with a 6-cell battery.