My impression is that the Chieftain's armor is better sloped and thicker, but it's still not likely to be proof against the Sov guns using solid shot from 115mm on up and will be vulnerable to HEAT from Sov tanks from 100mm on up.
From what I see in game, not having the mini-me turret will save some hits but tanks from both sides can get hits with no problems, particularly when they have laser RFs.
I really want to see what Chieftain brings to the table in game, but I don't want to get my hopes up too much that it will be the slayer of T64s.
H