Logically going, I place my bet on sturmtiger.
But logically jackill is right. The ISU and the Sturmpanzer IV use the "same" howitzers, why shouldn't they be HE. And the Sturmtiger uses an even bigger howitzer/mortar (with rocket grenades as I just read).
For mor depth they could introduce an option for the assault guns (Stug, SU-76) to switch between HE and AT grenades. Though the SU has already the barrage for that and the Stug the HEAT round, so...