According to Wikipedia, Flash has been in existence since 1996 and, because of this, has a strong base of developers and users. According to Adobe statistics, Flash has reached 99% penetration on Web browsers, while HTML 5 has a 40% penetration.
The latest version of the Adobe Flash Player runs on Microsoft Windows, Mac OS X, Linux, Android 2.2+, RIM QNX and Google TV. Earlier versions run on PlayStation 3 (Flash 9), and PSP (Flash 6). Adobe Flash Lite runs on Wii, Symbian, Maemo Linux, Windows Mobile, and Chumby. In contrast, some video sites, including YouTube, and blip.tv, have implemented a degree of experimental support for HTML5 video. Steve Jobs noted that Flash is not an open standard – it is controlled by Adobe Systems, whereas HTML5 is largely controlled by a committee (WHATWG) made up of three companies: Opera Software, the Mozilla Foundation, and Apple.
Some users have complained about the relatively high CPU usage of Flash for video playback. This was partially because the Flash plugin did not use the GPU to render video. Adobe has responded to some of those criticisms in the 10.1 and 10.2 releases of the Flash plugin by offloading H.264 video decoding to dedicated hardware and by introducing a new video API called Stage Video. In addition, the use of the newer ActionScript 3.0 inside Flash movies instead of the older ActionScript 2.0 improves code execution speed by a factor of around 10. But older websites that use ActionScript 2.0 will not benefit from this. Another reason for poor Flash performance is that some Flash developers incorrectly code their Flash files which can be a problem with “HTML5” animations as well.
To help you decide between the two, OneMoreLevel.com has crafted a side-by-side comparison of the two.