There is slight difference - Puma can reliably fight meds despite low pen, while AEC can help against them, but certainly can't fight them off by itself despite better long range pen.
They are same archetype, but their performance and potential differs massively.
AEC is better support for and against infantry, while Puma is actually a light tank destroyer
The Puma can't really reliably defeat medium tanks by itself without a lot of veterancy. But they've both got the firepower to penetrate a medium and, in pairs or with ATG support,
can win that fight. They're enough of a threat that a medium has to respect them.
The Stuart, despite ostensibly being an AT unit, is pretty much helpless. The best it can do is blow a load of munitions on Shell Shock and run away.
If the AEC and the Stuart were in the same tier, I don't think you'd ever see Stuarts.