Silverlight will just not play H264 content : as usual, microsoft has adopted a look alike, incompatible video format : VC1.
About why Quicktime is better that Flash when it comes to serious H264 usage, you may want to have a look at the following note/demonstration of a quicktime+javascript player :
http://blog.vrarchitect.net/post/200...ter-than-FlashIn short : Quicktime can reach any frame of a video. Flash just reach the I-Frames. So if you have a GOP/keyframing of 250 for instance, you can see only one frame every 10s of video (to be honest, most classical gop implies a frame every one or two seconds)