Krusty had the answer: where the glacis and the bottom come together on the real T34 is an enormously thick casting that should be nearly invulnerable. In AH that casting is modeled as the same thickness as the glacis armor, but at 90 degrees, so it is very weak. Hit that point and you will kill a T34 with one shot 100% of the time.
A T34 can kill a Tiger only from a range of about 400 and less and only if you hit either the side of the turret at 90 degrees (will take out turret only) or the side of the hull (where the cross is) again at 90 degrees (one shot will destroy the Tiger). If the Tiger is pointing at you it is entirely hopeless unless (as one unfortunate Tiger driver did to me recently) he turns his turret the wrong way and presents the side to you. If you are beyond 400 yds, never fire at a Tiger- it will just annoy him.
Edit: This is with HVAP rounds of course. AP will do nothing.