you're forgetting the setup time for schreks, which means that if a tank manages to get inside the squad the setup time and relic pathing dictate that the whole squad will almost certaintly get annihiliated as they keep running around and dying instead of actually firing the damn gun.
Not to mention that its not as easy to say that one has acc and pen the other has dmg so voila! Balance attained.
RRs squads, particularly bc of the fire up ability, can chase vehicles, schreks cant. Their accuracy at long range is abysmal and the setup time means that by the time they get setup the tank is out of range (if it is retreating). Setup time also leads, if I remember correctly to a slower ROF. Ultimately this makes the schrek only really good at close range AT, which means it must be supported by a pak (or more likely, be supporting a pak) whilst RRs can and very often are used as an purpose AT.
To illustrate the point:
http://www.youtube.com/watch?v=VjjN7DP4Yq0