It's true that the PIV armor increase is pretty good, as is Blitzkrieg. The Cromwell gets a faster reload at both Vet 2 and 3 instead, giving it a reload time of 5,93*0,7*0,8=3,3208 seconds. That's a pretty massive decrease. The smoke shell is also useful, and it can get emergency war speed as well.
The Sherman has almost the same AT performance as the Cromwell, with 140/120/100 vs. 135/120/105 pen and the same accuracy and reload time. It probably has better AI performance with HE ammo. But like the PIV, it's also easier to hit and less mobile than the Cromwell. All in all, I think the Cromwell is a bit better than the Sherman.
Sherman would have a better AI performance, if it wouldn't be shooting kilometers away of it's target. While cromwell is a good sniper. Of course RNG sometimes can mess him up, but...