Both are very nice and you should be happy either way.
Nook- expandable memory using microSD cards, has a pretty nice "read to me" feature that works well for kids books
Fire- I feel it is easier to use, a little faster processor, extra storage available through the
Amazon cloud, long term may have more apps because it sells more units, Amazon Prime is nice although it is $80/year
Like I said above, everyone I talk to that owns either one is happy. They are both good for web browsing, they can both stream Netflix, play games, read books and mags. The content prices are very comparable.
Hope this helps!