For the armor/helmet, is it better to get a UV that has a status resistance (shock, fire, etc.) or defense against damage types (shadow, elemental, etc.)?
The only thing that decides which is better is what the armor is and what you're using it for. There's a few exceptions in certain situations though; Piercing is regarded as the least useful defense, sleep resist is useless, Curse resist is situational. I'm sure having Max Shock resist on both pieces of Mercurial Mail would also be useless. Other than those types of situations there's not really much to go off when deciding what's better unless there's a physical choice between 2 specific UVs.
The only thing that decides which is better is what the armor is and what you're using it for. There's a few exceptions in certain situations though; Piercing is regarded as the least useful defense, sleep resist is useless, Curse resist is situational. I'm sure having Max Shock resist on both pieces of Mercurial Mail would also be useless. Other than those types of situations there's not really much to go off when deciding what's better unless there's a physical choice between 2 specific UVs.