For windows users .wmv is

For windows users .wmv is considerably more flexible than .avi. The problem with avi is that there are so many different codecs that you could use to compress it and you can’t be sure that your users will have the one they need to view it. The better codecs, the ones that give the best compression/picture quality like xvid or divx won’t necessarily be on everyone’s machines.

The other issue has to do with whether you want your viewers to be able to download and play your video or watch it right in a web browser. For browser based display there are really only four options Flash video, Windows Media Video, Quicktime Video, and Real Video. In order of prominance on the web you have Flash Player, Quicktime Player, Windows Media Player, and Real player.