WM9 is currently going through the SMPTE standardization process and should be an open standard (VC9) like H.264 in the not-too-distant future. Microsoft says it will take about six more months.
The success (or lack thereof) of H.264 will probably depend on its licensing terms, which were not yet finalized the last I looked. Many aren't going to choose H.264 over VC9 if it costs twice as much.
As far as performance, both WM9 and H.264 (MPEG4 Part10) make use of similar technologies and are said to offer comparable performance. The DVD Forum gave the edge to WM9 in its HDTV quality tests, but H.264 received the second-most votes for top quality. Both substantially outperformed the current MPEG-4 standard, which couldn't hold a candle to either at HD resolutions.
Generally, shipping hardware that can decode H.264 can also decode WM9, because both employ similar approaches with roughly comparable complexity. My understanding is that the VOOM STB upgrade will support both H.264 and WM9, so they can choose whatever works best on their end (encoders).