Imagine some kind of angry birds or worms (think of physics simulations, means: Stability and speed will be needed)
How would you do it, when it comes to platform independent free to play player vs. player with in app purchases and premium accounts via internet?

Would you use a different codebase for every platform (PC, Mac, iPhone, iPad, Android phones...) or would you go the HTML5 way? HTML5 seems to be easier, but will there be performance / security problems?

If you need stability and speed do it in C, mosync distributes to all kinds of mobile platforms; from java ME to windows mobile 8 to android. Userinterface is less easy to make nice and smooth, since you are not using the native SDK's.

the most important is to make it as easily and quickly accesible
as possible.

There Flash and HTML5 have a big advantage over any
local download/install solution.

Even worse if the player has to download and setup some libraries.

I would look into HTML5 here. (with WebGL maybe)

The physics for an agry bird game does not need a lot
of performance. (When implemented well)

For mobile platforms the best is their native APPStore
solution.
(Java here for Android)

Thank you for your thoughts. Atm I´m fiddling around with cocos2d-x, which has also a HTML5 / Javascript branch.

I absolutely love Corona SDK...

I´ve done some tests and it seems that html5 in conjunction with box2d is to slow for the iPhone. So I´ve to do it the traditional way.

But for gaining attention it would be nice to have a browser version of the game. Keep in mind: It must have pvp functionality (one vs. one over the internet).
Which brings me to the next question: Client side or server side programming? PHP or HTML5 / Flash / Unity Web Player?

I begin to think that there are too much possibilities.
The bigger the choice, the harder it is to choose.

box2D is a bit bloated for javascript at the moment, but depending on what you want to archieve, it shouldn´t be too hard to write your own perfectly suited physics which runs fast enough.
I really hate the unity web player and flash is also dead to me since there is the possibility to do the same without plugin and probably faster in at least some aspects.
The only big problem with webgl is that it doesn´t run iOS. But for a relatively simple 2D game inline svg could work pretty well on any platform with HTML5.

For simple 2D, I would just write everything from scratch myself, as everything else is usually too much.

as far as client side rendering I dont even have an opinion, but for server side, if your doing it browser based, php-mysql makes a good combination for database and information serialization

If it has to be realtime, I would suggest node.js and socket.io. Would be A LOT faster than PHP and it also scales much much better.