Well, there are some windows media files that will not play on my (mac) computer, and I have the Flip4Mac plug in that is supposed allow macs to play windows media files. Anytime you use the WM video 9 Advanced codec (which is the best windows media codec in my opinion), it is going to give people on macs a problem. In fact when I use Sorenson 4.5 to encode windows media files it will actually give me a warning that the file will not play back on Macs if I use that codec....so that may be the problem I am running into with some windows media files. H 264 will play in quicktime 7 player which is a free download and will work on PCs or Macs. But the computer has to be a fairly new model to have the power to play back H 264....so those on older computers will not be able to play it back
Really when you are doing this, you have to kind of look at who your audience is and what kind of computers they are likely to have. People in the business field are likely to have PC's so I'd probably use the Windows media route. If I was sending it to people in the media/production/arts community you have to think about Macs a lot more as they have a pretty big share in those communities. No matter what you use, quicktime, windows media, flash, or real player media you are going to have someone who it doesn't work for. If I was you and I could swing it, I may do more than one version....one in windows media video V9 Advanced, one in H 264, and one in flash.....you can set up sorenson to encode all three of these on one job....of course that may be too much work in which case you just make your best choice and roll with it.