Stug is probably more efficient but that does not make it more cost efficient nor does it make SU-76 a bad TD.
OK, i say'd it was on par in terms of price to performance with the stug. It costs less so it also performs less.
Like what more do you want form a 75f vehicle
60 max range(stug has 50)
170 max range pen(stug has 180)
3.875-4.375 s Reload(stug has 4.5-5.5 s reload)
fuel difference with mediums p4 45 and p4j 65 (stug has t34 0, sherman and cromwell 20)
fuel difference with special mediums Panther 110 (stug has t34/85 40, easy8 55, sherman-76 35, comet 95)
I don't know for me they look on par in terms of bang for buck.
Stug is more cost efficient AT.
Assuming the TTK times as written above (7 and 5 shots vs medium for SU and Stug):
Stug needs 21,5 s to kill an average Allied medium (326 MP, 103 FU), yielding 15,16 MP and 4,79 FU per second.
SU needs 33,1 s to kill OST P4 -> 10,57 MP / 3,62 FU
This gives StuG higher damage potential of +43% MP and +32% FU, while costing the same MP, +20% FU and +25% POP. This advantage still remains if you subtract 2 seconds of the SU TTK because of higher range. It equals out in the matchip against a T34 and improves vs Sherman and Cromwell.
So unless you can regularly take a long range shot, the StuG is better. Not to mention that a close range StuG is still a threat. A close range SU is probably dead because the StuG comes with way better defensive stats.
I'll say that this is a quite theoretical calculation since vehicle combat is all or nothing. But still, the StuG counters Allied resources more efficiently than the SU76.