It has worse stats in almost every regard: veterancy
Panther:
Panther PzKpfw V Medium Tank
Unlocks the 'Blitzkrieg' ability
+10% armour +20% health, +40% weapon rotation speed
-30% reload, +20% rotation speed, +10% ac/de-celeration
Comet:
Comet Cruiser Tank
Fires White phosphorus instead of normal smoke,This phosphourus burns infantry units all the way to death,blocks vision,and stops vehicles from firing.
Turret horizontal movement spend increased by 35%,+30% accuracy
Speed increased by 30%,Crew will attack nearby targets with their grenades by default,acceleration and decelleration increased by 20%
White Phos that kills infantry, better accuracy, better speed and auto attack grenades don't really sound like a worse deal than just the stat buffs the Panther has.
Both are good tanks that focus on different things. It's like wondering why the Tiger I has better general performance than the specialized Jadgtiger.