Actually, I vote for the Sherman next for a very good gameplay reason.
It won't make the Panzer IV obsolete. If you really stop and think about it, only the best tank will ever be used by the players.
Another tank added to the game should probably be better in some areas, but worse in others.
The Sherman is faster, but worse armor and guns. They probably will be used for attacks, while Panzers are used for defense.
Hanz.