I'm w SackUp, I want the extra 6% from the sooted hand when defending here.

If button is unknown and not crazy, then it's less likely to be a steal, more likely to be legit. Against a reasonable range of raising hands you are a toss up to have the best hand at this point and you're out of position, but I think C/R following up with donk is the way to go. If he 3 bets you, I think I'd probably call so I dont look like a pussy and fold UI on the turn.