Should everyone get the Critical Ring?

As of right now, not really. As of maplestory 2.0 (planning way ahead), yes.

Though really, chance at stun vs chance at insta kill... I know what I'd want.

I'm going to challenge this.
With the new system, magic attack assumes greater importance, so the extra 4 magic attack of the magic ring will directly affect total damage; eg an extra 2% for someone with 200-ish magic attack.