Since you already got experience in the game and hull down.
If you go M6, it's like KV-1 but +1 tier up. Hull down and top tier it does extremely well, I actually like the T1 Heavy and M6. If I got 57% and 58% wins rates in them, you can do better than I will.
If the M4A3E2, it has good armor, another tank I enjoyed. Stick with stock turret and M1A1 gun. You can't use the M1A2 gun on stock turret.

Second turret loses 74mm of armor!
Hard to say, I think both were good but if you prefer mediums style tanks, M4A3E2 is the way to go.