The wavelength band of UV light produced by Flash is broader. The light from Flash appears to be white. Even though a lot of energy of Flash is going into VUV, there is still a lot of energy in visible light spectrum. The VUV light emitted from Ray Gun has a much narrower bandwidth, and is therefore safer.
Flash delivers strong bursts of ultraviolet. Ray gun deliveries a continuous stream of VUV at a high frequency. More VUV is delivered safer.
In angle mode, Ray Gun can produce a large amount of ozone, nitric oxide, as well as localized heat. All these factors combine to treat skin conditions much more effectively than contact mode alone.
Ray Gun forms a microscopic secondary ionization layer between the gas tube and the target skin. Current flows between the tube and the skin. You can increase this current by placing an earthing pad on the opposite side of the target area. This increases the amount of ozone and nitric oxide, and more importantly, allows pathogen-disabling compounds to travel deeper into the body to reach problem areas, without harming healthy cells.