Not much alike except both provide a form of surface protection. 303 is basically a UV (ultraviolet light) inhibitor, while ProtectAll is a cleaner/polish and pseudo-wax surface protector. 303 has a matte finish and is intended mostly for protecting semi-porous rubber and vinyl materials that are susceptible to UV. ProtectAll is primarily a clean & shine product that leaves a protective film to help shed dirt. Works best on non-porous surfaces, e.g. paint, metal, porcelain, corian, etc.
Yeah, 303 advertises that it protects any surface and even mentions that it is safe for vehicle paint and even internal engine parts. But being "safe" doesn't mean it is effective or helpful - just means it won't damage it.