Not to throw a spanner in the works but H.264 can also go in an FLV container so these things are not mutually exclusive.
The consideration here is that pseudostreaming for MP4 isn't near as good as for FLV. For short previews this isn't as much of an issue but if you look at a lot of large tubes they are using FLV container for streaming H.264 to Flash players, rather than MP4.
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