The only downside of Sturm compared to Avre is the lack of cannon.
Damage is 580 vs 440
Range is 40 vs 35
Deflection damage is 290 vs 220 (penetration is 1000)
AOE radius is 14 vs 8
Distance near/mid/far is 1/3.75/8 vs 2/4/8 (still with the higher AOE sturm wins)
Damage n/m/f: 580/145/44 vs 440/220/44
Even in friendly fire it wins, deals 10 times less damage on near than AVRE (29 vs 220)
Vet is also better on Sturm. Why do people complain about that unit when they can't use it... well, not surprising really. Most people have L2P issues. Screech on forum after losing one game.
Most of those supposed advantages for the Sturmtiger are irrelevant because the AVRE is already capable of wiping squads with pretty much every shot. Higher stats are overkill.
The good mobility + turret + 30s reload that begins immediately after firing are what makes the AVRE so good. The terrible mobility, casemate design and manual reload are what make the Sturmtiger so bad.