After using n9 for few months I decided to go back to n900.
Generally i have the feeling that n9 was big step back in comparison to N900.

I liked design, bigger amount of ram and better processor was nice, but i have found n900 still impossible to beat in terms of usability and functionality.

Main cons of n9
-no fm transmitter
-Gui not bad, but not so comfortable as hildon, my finger was tired after using it.
-Still lacks a lot of ported applications, some of them probably never will show on n9 (e.g. aircrack, preenv)
-Processor is not so much better than overclocked n900
-not to good flash player
-annoying stock browser
-less storage than in n900

N9 pros:
-internet radio application
-design + lighter than n900
-better RAM+CPU

So for me n900 is still best phone around
Just want to share with my thoughts