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.
Puma is a league above the AEC imo.
The fact that the puma doesn't need to be in return fire range is just so huge given both of their low durability.
Then the aimed shot for the puma just makes it such a superior support unit (it can even force team weapons to redeploy so it can support infantry or friendly armour)
Making the Stuart like an AEC might work, but I'm under no impression it would fill anything like the role it would if you instead mirrored the puma (not suggesting that by any means tho)