-fast startup at 3f
-easy, guaranteed blockstring on most characters via TC4
-shitload of time to hitconfirm
-TC4 is character specific, and spacing specific
-relatively low damage potential without Tsumuji loop
-lowers Tsumuji loop-ability
-good damage, and sets up potentially damaging combos (st.MP st.MP st.MK)
-sets up universal Tsumuji loop
-perfect pushback on block for kara throw setup
-combos into itself and auto frame traps
-very easy to option select afterward (specifically due to the lack of special cancellability)
-slightly slower startup at 5f
-does not net you a blockstring
-also character specific, spacing specific
It’s mostly a matter of personal preference, or the matchup dictates which should be used, and mostly what you’re trying to accomplish.
st.mp does more damage, leads to more damaging combos. For me, it’s what I can do afterwards that’s more important. I can go for the throw, I can go for the counter-hit setups, go low, take a slight step back and do a cr.RH, do a kara-throw, hit confirm into a combo, go for another cross-up, command dash mixups, etc etc.
But st.lp comes out fast, you can use it for jab pressure, it punishes certain moves on recovery, or before it lands, it counter-hits often against characters with slow pokes at close range, it pushes your opponents away, etc etc.
Ibuki is a character where it’s really your situation that defines which buttons are preferred over others.