buying books outside of Barnes and Noble is less of a hassel to do on the nook than buying books and converting them on the kindle.
The nook supports e-pub which is pretty much the industry standard which the Kindle still doesn't support. Yes as stated you can buy and convert but with the nook you can buy and sideload right then and there. Also the nook supporting the epub format allows you to check out books from the library.
another thing to consider is if there is a Barnes and Noble in your area, if so you get some extra fun features in the InStore Experience, meaning you get special coupons, deals, read any book in store for an hour, freebies from the StarBucks Cafe etc. A deal breaker, No a very nice added touch to us bookstore lovers? a BIG YES. Also you can get in store support if you have any problems or questions etc.
The nook does have a replacable battery, expandable memory. I love the color touchscreen navigation, color book cover flow and Free Fridays.
Now if you are big annotation and note taker the physical keyboard of the Kindle may be to your liking, if not i think imho it then becomes an eyeshore and wasted space but that's me.
Good luck and I'm sure you are going to love either one