Old HE shell had the worst DPS against vehicles by far, and terrible splash damage, worse than even sabot. It also had terrible velocity, which meant it could be easily dodged, and the high drop meant that at longer distances your damage would start to drop due to angle modifiers. The only real advantage it had was higher single hit damage, and thus larger crit zones, however this was almost entirely negated by any tank driver that understands angles.
The new HE shell has the best blast damage of the three shells, has faster reload and auto replenish so it can output that blast damage more often, and using shells against infantry is less costly if you get engaged by a vehicle shortly afterwards. It has the same velocity and drop as AP shell. Its DPS against vehicles is only marginally lower than AP/Sabot, and much higher than the old HE shell. Its biggest disadvantage is low single hit damage, which means crit zones are small and it can be difficult to land mobility hits.