it's only when you look things up it becomes more apparent how difficult the choices are....
The Su series of assault gun/tank destroyers came into being early 1943 after the Stugs were encountered so weren't at Stalingrad. They were designed to be used not as tank destroyers but as fortification busters hence the huge guns and the HE shells. Brute force as opposed to German accuracy and AP shells. Yes, they are powerful but are a pig to operate.
I agree, the Su and Tiger aren't an equivalent match but Ru doesn't have anything to kill Tigers. If anything, take out the Su and the Tiger.