Brass pros: Next to gold, it is the most beautiful whether polished or not; polishing is easy and rewarding whether the body is knurled or not; its weight lets you know you're not holding some flimsy trinket; its threads, when carefully machined as are Peaks, are silky smooth.
Brass cons: Drop it on a hard enough surface from high enough and it can be dented; larger brass lights may be a bit heavy for pocket carry; longer brass lights on a keychain can really bang your dashboard on a washboard road; kitchen smells like Brasso; everybody wants your light.
SS pros: SS looks far better than HAIII, and knurled SS Peaks are especially attractive; it can be polished (though not as easily as brass) and stays polished a long time; it has to be really abused before its appearance is damaged at all (and you would need a microscope to detect scratches or dings on a SS Peak); on smaller lights, its added weight is not an issue for keychain or pocket carry.
SS cons: It's not as beautiful as gold or brass; need to keep the threads lightly lubricated; no other cons that I can think of at the moment.
Whether any of the three metals will hold up longer depends on its use and abuse. If it will be used as a tool, select SS; for edc or casual use it probably doesn't matter. And just so I won't be accused of being entirely subjective, I would point out that all lights, whether constructed of HAIII, SS or beloved Brass, will fall in a vacuum at the same rate.
