Well, the one that worked was using MPEG-4 Video which is a codec that has been included in QuickTime since the release of version 6 (November of 2002) and the one that doesn't was using H.264 which is only part of QuickTime 7 (and was first released in January 2005).
Anyone without Quicktime 7 on their systems is not going to see your video if you use H.264.
The thing is, even though H.264 is based on MPEG-4, it is not MPEG-4... it is a derivative codec made by Apple.
Like I said, to reach the largest audience you shouldn't use the cutting edge file formats. You should aim back a ways, and supporting QuickTime 6 is a good way to do this.
Also, you don't need television frame rates for web video. Your movies are at 28.22 FPS (broadcast television is 29.97 FPS). Taking the frame rate down will let you increase the picture quality while keeping the file size small. Feature films run at 20-24 FPS... I've found that between 15 to 20 FPS works fine on the web and doesn't overly tax older systems.