I modded nerf blasters for a long time, and recently I have been modding airsoft guns. Here are the differences that I see.
The Mind Set
Nerf modders do not care about realism - Well, not as much as the airsoft community - but Airsoft players like the realism. I find it strange to expend some of your limited resources of green-gas to move the slide, when you can use that to further propel the ammunition. Nerf players do enjoy realism at times, but they understand that, ceteris paribus: victory goes to the one with the better gun. Form over function.
Nerf Titan AS - V1
On the front lines of the foam wars, this was a weapon to be feared. After some modifications, the energy used to propel the heavy rocket could be used to jettison a single dart. It was never a mod for accuracy, but it had the ability to shed blood and knock over children.
Parts
Did you want a new spring to upgrade your M4? You go back to the airsoft store and buy one. If you want a spring to upgrade your Retaliator, you have to either buy it from a third party specialty seller, or you can accidentally find one. There is a certain enjoyment of exploration when modding nerf. Sometimes you find a motor upgrade from a robotics retailer in Europe. Sometimes you discover a suitable O-ring from Home Depot. However, everything you want for airsoft is usually found at the same place you got the gun.
A link to the blog of a friend of mine that I once played nerf with. His mods are well respected in the nerf community, and are sold on his ebay.
Safety
I’ll be honest. Airsoft is more safe than Nerf since rules are usually arbitrated by the host. Airsoft has fixed rules and respect for the guns, while Nerf does not as much. Anything that is homemade is likely to be allowed in Nerf, from foam bashable shields to modified 40 psi fire extinguishers. Airsoft requires a maximum feet-per-second for your shots. Sometimes they won’t even allow melee weapons.