Gruul Ponza's ramp creatures are mostly small (Arbor Elf, and the Eldrazi Spawn tokens from Malevolent Rumble and Eldrazi Repurposer), so Arms of Hadar's -2/-2 to all their creatures cleanly sweeps that early mana base and buys time before Annoyed Altisaur or Boarding Party come down. Grim Bauble's -2/-2 only hits one creature for a similar mana investment, so the sweeper effect of Arms of Hadar is strictly better value here. Keep the rest of your removal suite, including Cast Down, since it is still needed for their bigger threats like Avenging Hunter and Boulderbranch Golem once they resolve. Watch out for their land destruction (Mwonvuli Acid-Moss, Thermokarst) slowing your own curve.