I played both around the same time frame. So, I don't have nostalgia toward either.
New Vegas has aged much more well. I like how there is more locations to find where as in FO3 you can walk 2 miles without finding anything. The writing it also better in it (especially in the DLC's) and the combat in it was also improved from 3.
There is actually a whole list of things I like about it, but I simplified my opinion in this post for the sake of me not wanting to type huge paragraphs. So what do you think has aged better NG?