When you say practical fire-rate, what exactly do you mean?
Also, doesn't the Stug get a bunch of RoF upgrades from veteran levels?
Just mean the actual rate of rounds fired, as the actual thing broken down is a bit complex in total. Point is, in terms of how many rounds they'll be able to shoot at a still target in range in a given amount of time, the Pak 40 will probably be a bit more.
The StuG G's reload bonuses from vet are alike to the Pak 40s (Both gain 0.7 and 0.9 multipliers, though the StuG G get the 0.9 from vet 2 instead of vet 3). The differences from there are that the Pak 40 gains 1.3 penetration and 1.3 rotation, while the StuG G gets 1.2 rotation, speed and (de)acceleration.