IMO,WP7 will surely die, even if the sales are good.

Windows 8 is coming out and it will support both Intel and Arm. Microsoft will almost surely kill it's own WP platform in order to get better sales on Windows 8 (Or whatever called).

Unless there are no discrepancy between these 2.
Really? You want to know more about WP7? Well, there is a review of Mango beta build out there.

But my recommendation is: If you want to tie up yourself, there is a much better choice.That is,the iPhone 5.

The author said that her HTC trophy was smoother than her Droid. So the fact that the WP7 is smooth would be true only if you compare it's hardware with the 2010 phones. And not phones like Samsung Galaxy SII, which is fast. And iPhone 5, which is even faster.

The reasons I chose the N9 over the others are: The openness, e.g. possibility of the emulators, and the fact that they are not going to be kicked from the repos ( N9 most likely has one). And they are not going to lock you up to their store and feeding you recklessly just like the others. For me, N9 is going to be smoothest and fastest Linux ever, for sure.

P.S: Also I wanted to tell you that the flash player for android is a really crippled one. Even the flash player 11 beta, which is apparently designed for the IE10(and other windows and Mac browsers), performs really much better than the android one(IMO). So don't need to be feared that N9 has none.