Nokia is distributing the N950 to developers through a commercial (Nokia Launchpad) and a community (MeeGo Community Device Program) channel.

The N950 is not more advanced in terms of specifications.
It does not have NFC connectivity but on the other side it does have a 12 mpix camera.

Both the N950 and the N9 are using the same SoC and mostly the same hardware. Both are running the same OS (MeeGo 1.2 Harmattan).

The same apps will run on both devices.

edit:sjgadsby was faster...