Lethal Radius on the SU is 1.5, Lethal Radius on the Scott is 1.35.
Su-76 has a radius of 5 compared to the Scott's radius of 4.
SU-76 reloads faster.
However, much more importantly, the barrage is an AI upgrade to the SU-76's gun.
The barrage is an AI downgrade to the Scott, which can otherwise autofire at infantry as they move.
None of this is very important to the fact that the Scott compares very poorly to the SU-76 because they are a AT/AI unit respectively.
That does not justify you description of the Scott's barrage as "useless".
The barrage is inline with other units (better then mortars/min howizter ) and compared to SU-76 it can barrage on the move and provide smoke barrage. It useful in countering ATGs or static emplacement.
I would trade the TWP ability of the Stug-E for a barrage similar to the Scott any day.
Finally the unit is OP, it very hard to counter and does lots of damage.