HTML5 video and audio is certainly more cpu efficient, and thus in today's world of mobile devices, preferable IMHO.
There's a lot of cool stuff you can do with CSS3 and HTML5 that required Flash in the past, but Flash is still very, very popular and is still being used on a large scale to create content for the web and even for iOS devices... (really!)
Flash pretty much had a monopoly on interactive and rich-media until now... and still is easy to use, whereas HTML5 content creation software like Hype needs more time to mature.
Just my $0.02