I have a nook and a Sony 650. Both are great readers! But if I were making your choice today, I'd take the 950 over the nook--even with the $100 price differential.
I know the Sony 950 interface is basically the same as the 650, but with the addition of wi-fi. Personally, I think the Sony's interface beats the nook's hands down--even with the nook's recent upgrade.
One reason I decided to upgrade to the nook from my Sony 505 was for a dictionary. But accessing it on the nook was very cumbersome--having to use the lcd screen at the bottom to navigate through the document was onerous and there is no way to look up words if you don't know the exact spelling. On the Sony, you can just double tap the word you want to look up and the definition pops up on the bottom of the screen. You then have the option to see a more detailed definition and to scroll through the dictionary.
I really like other aspects of the Sony Interface as well--IMO they've made everything very intuitive--and I really like the touch screen interface. The nook's interface isn't too bad once you get used to it, but the Sony's is much better.
Other pluses for the Sony--battery life and weight. Here I'm making an assumption that the battery life of the 950 with wireless turned off will be about the same as the battery life of the 650. Nook Battery life supposedly was improved with the recent upgrade, and I do see that to a small extent. Still I get over three weeks with my Sony (and I've never let my battery get below two bars before charging). On the nook I'm getting about a week and a half now. Was less than a week before the upgrade.
And on the weight issue--the Sony 950 is far lighter than the nook (I compared this at a Sony Style Store.) The nook and the Sony 950 are almost the same size overall, but the Sony has a larger reading screen and no LCD screen which I'm sure adds the weight to the nook.
Don't get me wrong, the nook is a great reader--but in my opinion the Sony beats it hands down (except of course for price)!
I agree with jocampo--visiting a store where you can compare the readers is a great idea. Also, if you have some books on an SD card, you can just plug that in to the floor model of the Sony's to see how your own books would look. Not so easy on the nooks as you have to remove the back of the reader to insert the micro-sd card. (And I'm not sure if the books need to be in a special folder for the nook--no such restrictions for the Sony).
Good luck with your decision! Either way, I'm sure you'll be happy with your new reader!