I think you should try a wp7 in the store or from a friend first.
I think when it comes to speed you'll definately notice the improvement compared to the n9.

Wp8 will only be better.

Totally depends what you want with your phone.

I am most likely exchanging the n9 for a wp8 (lumia 920).
If i look at all the features that come with it out of the box (kids corner, data sense, wordflow keyboard), and how its integrated with social networks, email, calendar, skydrive etc. wp8 would be my choice.