The Stug´s main purpose in the early days was as a infantry support weapon, the early Stug was only armed with a short 7.5cm StuK L/24 gun.
The late war model (Stug IIIG) was armed with the long StuK 40(L/48) gun, and that really paid of, now it became a potentiel tank killer.
The Jagdpanzer 38 Hetzer was a real anti tank weapon and only build for that purpose.
Which you can se on it´s gun a 7.5 cm PaK 39 L/48.
And a little further information, the crew of the Stug was recruited in the ranks of the artillery, and not the Panzer, and it was one of the only ways for artillery men to distinguish themself in combat.
For more information read Panzer leader, by Heinz Guderian.
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In god we trust, the rest we monitor...
von Schalburg