The bottom line for many airsoft pistol enthusiasts is the fact that GBBs are a lot more authentic and much more fun to shoot. That's about all that has to be said if this represents your preferences.
Nevertheless, you will find other differences also which some might wish to consider when determining which airsoft pistol to choose.
Let's start with price. GBBs are generally more costly than NBBs. This tends to make sense. The GBBs usually are better constructed, made from better supplies, and are much more realistic.
Loading and cleaning. GBBs tend to be simpler to load and simpler to clean. NBBs often have internal magazines which are difficult to load.
CO2 use. NBBs tend to use less CO2 than GBBs. They do not seem to leak as much as GBBs. Obviously, certainly one of the reasons GBBs use additional CO2 is because some of the energy will go towards cycling and reloading, whereas with an NBB, all of the energy will go into moving the projectile.
Sound. NBBs are quieter than GBBs. If you want the realistic sound, you're going to go with the GBB. If you do not care about the sound or want a more stealth strategy, the NBB will accommodate you.
Accuracy. I'm not aware of any research demonstrating which model is more accurate. Nevertheless, one crucial distinction is that NBBs have less moving parts than the GBBs. Less moving parts in the course of a shot can translate in to improved precision.
Realism. The GBBs have a tendency to be more realistic in physical appearance, performance, as well as sound. For example, the blow-back slide tries to duplicate the action of a genuine gun. In general, this realism is the reason why GBBs tend to be more pricey than NBBs.
Upkeep. The GBBs tend to require more maintenance because of the fact that they have more moving parts. You are able to take down the GBBs much quicker than the NBBs. The GBBs are also simpler to upgrade than the NBBs.
Overall performance. It is true that NBBs have a tendency to possess a slightly higher Feet per second than GBBs. This is minor, and again, it is a trade off with the other areas outlined.
The bottom line is that it all will come down to personal preference and what you want from your airsoft pistol. GBBs really are a joy to shoot. They have the recoil, the sound, and the look. NBBs are cheaper and may have a slight edge in a few areas such as CO2 use. Most airsoft pistol aficionados tend to go for your GBBs simply because the reason they got into airsoft to begin with was because of the authentic nature of the guns as well as the fun they get from shooting them.