In that case - I had one more thing I thought of to add. While it looks equally good on your CV to have either grant - a thing to remember is that usually when you give talks etc. in school your PI will introduce you as a NSF or NDSEG fellow. The grant will also be on every poster you present and every talk you give. So if you think one is more prestigious/has more name recognition there is a realistic reason to choose that one. I vote NSF