One of the primary differences in NBB vs. GBB is that the GBB is more realistic in all ways, this also includes accuracy. There's a reason soldiers only use sidearms as backups except in very specific conditions: it's damn hard to hit anything at any range with a pistol, especially in any kind of rapid fire, because the blowback throws your aim off. My group had a pistol only game recently and I tell you, I had a hell of a time hitting anything. But as I said, they're realistic. If you want efficiency and accuracy and long life with little maintenance then get a NBB, if you want realism then get a GBB.
Personally a would recomend a GBB. GBB's are a hellofa lot of fun to shoot and as long as you're only using them as a backup/close-range weapon rather then a primary then the above mentioned points in favor of NBB's don't have a lot of impact.