The stug/su76 comparison is really apples to oranges... they arent directly comparable as they both have different roles... the stug is meant to be dedicated AT while the su76 is supposed to be stopgap AT to transition to firesupport AI...
The su76 does the former well enough imo... what it lacks is the latter
For LV counter SU always have much more cheaper variants (ptrs penals, or at-nades with zis). For medium counter - i already told it's not role of su-76. I don't even could imagine situation when in normal game SU player will be build first unit in T3 SU-76 not T-70. Sitting in T3 also meant that game for SU player will be almost ended. You can't counter P4 with SU-76, while any delay of T4 also delay your heavies and normal armor units. If you will lose your SU-76 you will be without ANY AT (if you have zis, you don't need su-76 at all) and need to build new one or make backteching to zis.
That why i want change role of SU-76. If it will be AI support unit - player will have 2 potent variants:
1. Aggressive, risky T-70.
2. More defense-style Su-76.
While now - no one in entire univierse don't say that you must build SU-76, if you have zis.