Long story short for PvE DPS:

1) Pick either max mag or max stam, because your skills will scale with whichever's highest. All you need is enough recovery to use the mix of skills you have. For example, there are stam arcanists who stack max mag because they use Mages Guild skills
2) You can mix light and medium armor in any amount, you don't need 5 of either for the passives any more. You still might want 6-1 or 7-0
3) Most non-hybrid builds use 1-2 skills in their off-stat anyway, because it helps with sustain and lets you use the highest-damage skills

Whichever is higher between the two will always be used: max stam vs max mag, weapon damage vs spell damage, weapon crit vs spell crit. All weapons scale with either, so a lot of stam AND mag builds use frontbar daggers and backbar inferno staff.

Agree with @tsaescishoeshiner

I go all stam or all mag based on what I expect to use as a spam skill or the weapon(s) I'll use. Then I ignore mag/stam regen by relying on a mix of skills that does not over tax either. That allows more focus toward crit, spell/mag damage, etc.

I use the 3m dummy and world bosses to evaluate the effectiveness and sustain of my skill mix and fine tune the mix to work with my no mag/stam sustain approach.

For example, I run a stamina Oakenbow warden who uses three mag and two stam skills. One of the stam skills is a 'spammable' but the other four skills are all over time and don't need rapid casting.