Hi,
I just wanted to present my point of view on this issue in regards to HTML5 versus flash. I ran accross an website that has a pong table setup with 1/2 flash generated and 1/2 html5 generated. When you run this website in Safari (the most recent version) the flash video is choppy when the ball crosses the screen. When the ball crosses to the HTML5 side it is very smooth. This is what my expectations were for what I have been hearing.
NOW, this is where it gets interesting. I took this website into Firefox and ran the same test. The ball had the same performance in HTML5 vs. Flash. The ball was faster in both and choppy in both (meaning the video performance was better but still choppy).
So my conclusion is that one factor is the actual engine (flash or HTML5) but it also is dependent on the web browser and how it is programmed to interpret the flash/html code efficiently. I draw this conclusion because I believe that Safari is programmed to use HTML5 efficiently and flash very inefficiently. And firefox is programmed to do both somewhat efficiently but not as good as Safari on the HTML5 portion.
Test for yourselves:
http://labs.codecomputerlove.com/FlashVsHtml5/