Buy a pair of trousers with large pockets and keep both, like I will do!
It is very clear that those who love the N900 will miss a lot of its features in the N9.

Enjoy the N9 as a phone and for its large RAM upgrade mutitasking browser performance, its nice looking screen, integrated always online IM features,... to shut op iPhone users when they laugh with your brick like N900...

Use N900 for anything that needs a keyboard or precission input, drawing etc...
N9 screen and virtual keyboard will drive you mental in a stress situation when you were used to productivity on N900.

The N950 we leave out of consideration now as it is not available.

In the years after Elopcolaps of Nokia, keep both N9 and N900 and if you are lucky enought to find a N950, that one as well as vintage items next to ZX81 and Commodore 64 and AMD K7 with white box Asus motherboard that did not exist.