i have questhelper on main. monkey quest helps to see the alt's quests progression, but if the quests are identical, then i dont think its necessary. EQL (extended quest log) to see better.

button mods on my main and slave. why? sometimes i just need to mouse over on my slave to press a key that's not hotkeyed. making the key 3x bigger on the slaves makes it pretty easy to click on from its PiP size.

unit frames only on my main.

as for the choices between different mods that do the same thing, i cant really say. i only use certain ones and do not have much knowledge of every competing addon out there.