As long as the tank maps provide cover and concealement, then the Stug has a chance, but in a knock-down, one on one fight, the IS will clean its clock. The German team will have to make sure that whichever players are operating the Stugs understand the importance of firing at optimum range and from optimum angles. I.e. don't expect to fire at the front of a IS tank and get results, but hiding unnoticed and coming in from the sides or rear is another story.
IS players will probably have to watch out too, because if they are too hasty and fire off a round (or try to fire while moving, which I see lots of players do in the mod), then they will be stuck reloading for the next 30 seconds while the Stug pumps several rounds into it
Hopefully the devs have also taken into account realistic turret traversing speeds, which will present further dangers to the IS if caught in the flank or rear.
In general, if the game tactics play out like anything in real life, the key to the German success will be taking the smaller numbers they have, and concentrating on select targets, such as having two Stugs take on one T-34, with one firing while the other reloads, or the use of flanking maneuvers and feinting to get at the Russian armor's flanks and rear.
The Russian team on the other hand, will have to mind these tactics, and in turn assault German positions with speed (from the T-34s) and large amounts of supporting suppressive fire (from the IS and SUs): the quicker they can get in amongst the Germans, they will be at an advantage in a highly mobile fight, but at distance they may all be taken out piecemeal by a concealed Tiger or group of well-positioned Stugs. Of course, when an all-out assault is required to finish the job, a high-speed surge of a large number of vehicles all at once (with JS and KVs leading) stands a chance of overwhelming German defenders.
Hopefully we will see realistic differences among the vehicles, and with that, the players who are serious about winning will use teamwork and historical tactics. I am also hoping VOIP is a little more flesched out in the retail version, which would help a lot with coordinating armor attacks.