Your choice of tools will be informed by the type of game you want to make. For example, if you're looking to so something with 3D or something that requires broad audio support, you would be wise to steer clear of HTML5 -- lessons learned when I was building Teletoon's Toon Feud game. However, if you're building something simpler that doesn't require excellent audio support and synchronization, across-the-board 3D support or peer-to-peer networking, and are willing to work around occasional browser incompatibilities, HTML5 is a good choice. I have no doubt that HTML5 will continue to evolve so in a few years we can expect it to have the capabilities of Flash 8 -- just not there yet. I broke it down in greater detail here: http://patrickbay.ca/blog/?p=153
If you're looking for a broad platform (your game can run on as many devices as possible without having to alter your code), I highly recommend Adobe Flash/AIR. I've developed nearly 90% of the games I've worked on in Flash or AIR and have no regrets. Keep in mind that your audience probably won't care what underlying technology is being used so the important take-away is that you work backwards from them -- what are your audience looking for? How will your game achieve these things? How can your choice of technology make this happen? Most successful developers don't tout the underlying technology (except to interested nerds), they tout what their product does for you.