CleverNickName
Limit Break Dancing
Yeah, I'm leaning toward using "10 + Dex mod" just out of simplicity.I'd personally make it a Dex save against a DC of 10 + attack's mod, to keep in theme with the idea of dodging an attack that needs only touch to inflict an effect.
Or yeah, if you want to keep it as an ''attacker rolls'' thing, attack roll against 10 + dex might be the easiest way of doing it.
I'd love the "dexterity save" option, too, if it weren't for the various abilities in 5E that trigger and/or negate based on dexterity saves. And that's fine, except when it doesn't make sense (for example, using the Shield Master feat to cause that attack to hit your shield instead and therefore miss your shield. Somehow.)
I briefly considered just giving the creature a discrete bonus or Advantage on attack rolls against armored targets, since (as @humble minion pointed out) there are a ton of different bonuses added to AC besides just armor or shields. But it feels really complicated and fiddly, and that's why I'm soliciting feedback from folks who might have done this before. Where did you draw the line? Which bonuses did you use and which did you omit?