Same problem for me. I'm tending towards NDSEG, because
1. my field funding situation is pretty good and my GRFP choice is unlikely to affect my ability to get funding,
2. NDSEG is slightly more exclusive, and
3. NDSEG does not publicly announce the possible awardees, while NSF does - you only get both resume items officially if you accept NDSEG.
Your mileage, of course, may vary.