T-70 can do much more damage, because it actually can hit what it shoots at.
And going for T3 will be a suicide if you haven't dominated early game, because you'll end up with T34 and IS-2 while your opponent will have a pair of Tigers at the same time.
Your first T34 will cost only slightly less then Tiger and doesn't have half the impact of T-70 or even fast P4, which again, can actually hit what it shoots at.
T34 is good to chase remaining light vehicle ( or even a StugE, good luck with a T70), circle/crush squads, and can resist way more than a T70, if at 6 CP you have a T34, you can screw up an axis player waiting for 11 CP, gaining map control, so more fuel, will give you more heavies.
Of course, i don't recommand going T34 if you just lose early and mid game, and i never did
T34 its a great tool to "push even more the pressure" and punish axis player waiting only for heavies, anyway you will have heavies too in late game