From my point of view:

N950:

- Bigger size, odd handling, feels like a previous-generation phone to me (especially after using the N9 for a while).
- I wouldn't call the hinge flimsy, but opening it is a little awkward. I personally don't like it.
- The hardware keyboard feels a bit out of place with the mostly portrait-style OS (and really good VKB), but is otherwise the one advantage it has.

N9:

- Smaller, has a gorgeous and solid unibody design, fits perfectly into one hand (which makes a lot of sense with the portrait-focus of the OS).
- Has a much better screen.
- Works a lot better with the swipe UI, both because swipe works best when held in portrait, and because of the curved screen and bezel.
- Has NFC

Everything else being pretty much equal, I would say the N9 wins this one easily. Of course some are simply not looking for an actual phone like the N9, but that is a different story and has little to do with the objective merits of the two devices.

If you are looking for something different, call it mobile computer, mini-netbook, whatever, then the N950 is obviously closer to that, and not everybody cares about ergonomic and aesthetic features of the hardware. But frankly I think that the N9 excels far more at being a phone than the N950 does at being a mobile computer or anything else really.

The N950 still a good device of course, just not exceptionally awesome (as it should be).