The NSF lets you defer for up to 2 years. That could be important if you are covered on a training grant for the first couple of years of your fellowship (you would be fully supported for longer). The $10k is 5k to you and 5k for "research", which depending on the field may or may not be relevant. The extra $5k to you is a lot, percentage wise (although the stipend of the NSF changes each year).
FWIW I chose the NSF after talking to my advisor and a couple of other professors whose opinions I trust. They thought that the NSF was more prestigious, mostly because it had been around longer. My professor was happy to have me covered through 6 years, since I am on the training grant for the first three. That more than covered the extra $15k I would have gotten for research. I didn't really need the extra salary, and I was a bit weirded out about the travel restrictions, and so all in all thought that the nsf was a better choice for me.
It's a pretty good problem to have...
