P4 have 3 MGs and can shoot planes.
34/85 have lower scatter.
P4 have more armor and will deflect shots more often.
34/85 have hp to withstand 1 more shot.
P4 have much better vet ability and can acquire awesome doctrinal ability.
34/85 is doctrinal, P4 is not.
1) The MGs on the T34-85 deal more damage than the P4 MGs and are free.
2) T34-85s have higher penetration and higher HP than P4 but lower armor.
3) 85s are better against infantry.
4) Being doctrinal is an advantage, not a disadvantage.
T34-85s are therefore MUCH better against Tigers than P4s are against IS-2s since both the 85 and P4 will be penetrated 100% of the time by the IS-2/Tiger.