vs a UTG open off 15bb or so, I am likely just folding A5s, flatting imo is the worst of the options, BUT depending on player type, I like a 3bet more than a jam for 30bb effective(button has same stack as you, still to act). You could 3bet to say 6 or 6.5bb and easily fold if the button 4bets you, and either take down the dead money if he folds, or play post flop with an uncapped strong range in position. It would be a disaster if you 3bet jammed 30bb and the button wakes up with QQ+ AK maybe even AQs and snaps you off. If you 3bet, you can still get away from the hand fairly cheap.
If the utg player is tight and or a nit, i'm folding without a second thought as if you think about it, he is opening UTG (strong range) off of a 15bb stack (filled with Ax type hands that dominate your A5).
If the UTG player is a loose fish playing 45/20/2 type stats and opens something ridiculous like 25-30% from utg/EP, and has a reasonable fold to 3bet stat, then I like a 3bet.
So I rank folding 1st, 3betting to 6.5ish BB 2nd, jamming 30bb 3rd and flatting 4th.
Now on the turn is an easy fold to the jam. You are never ahead and best case drawing to a 3 outer with a five. They will almost always roll over an Ace with a better kicker or AQ/77.