The most important bits, if you don't have them don't use the guns! Here's a couple of pointers:
Masks can be claustrophobic for some people and they generally make you a bit sweaty around the face and can restrict your breathing slightly. They restrict breathing because the inside of the mask always holds some of the CO2 you are exhaling. The major benefit of masks is that they will prevent your teeth from being shot out. A solid BB fired directly off a tooth WILL more often than not break your tooth.
Glasses are less restrictive BUT increase the chance of tooth-loss, they do not cover the eyes as well as goggles due to the gaps around the sides and unless you use an anti-fog agent, are prone to fogging. That said, many airsofters play for years wearing glasses without any incidents.
Goggles are less restrictive than a full face mask but offer great protection for the eyes. You will again be gambling with your teeth and the tooth fairy may receive one or two sooner than you intended. Unless the lenses are mesh, they too will require an anti-fog agent to stop them from steaming up.