if your only restriction is time then go with flex, you will be able to export your app as a native iphone app, will work on browser via flash and on android, desktops (linux, win, mac) via adobe air
developing with html5 is not trivial, I agree that there are awesome jquery plugins, but developing with drag drop and drawing is not the way to go with html5 also with html5 you can have cross browser issues for web2py you can use remoteobject with @servise.amf3rpc I have kind of cms in flex (administration part) for my website ( http://www.fermer.md) and it works great with web2py and pyamf http://dev.sencha.com/deploy/ext-4.0.0/examples/sandbox/sandbox.html this looks like a desktop app, and flex is used both for RIA (flash) and desktop (AIR)
