Hetzer sucks so bad. It is slow and cannot mount a gun that can reliably penetrate the heavies. Get 105mm gun then it'll suck less but still suck. I killed more in my shorter career on Marder II than my long grind on Hetzer (still going!). Marder II often would be placed in a game that has lights and mediums, making it easy kill for me.
I noticed something slightly similar with the BT-7 and the A-20. I don't know for sure, since I didn't spend ALL that much time with the BT-7, but my damage taken/given ratio is much better in the BT-7 than it is in the A-20, even with the A-20 nearly fully upgraded. (Just working on the radio and one last gun that I don't really intend to use, just getting it for Elite.) I love the A-20, and it's speed, but I think, once i get it upgraded, I might actually save up and buy a BT-7 again to give it some more time on the field.
Now, I'm just now starting to learn how to play light tanks. So those results could have been a fluke, or good luck. I'm actually playing them more as scouts now than as a participant in battle. I'll find a good place to hide off to the side and just let the enemy roll on by me as I reveal their positions to the heavies and arty behind me. Once things mix up a bit and the battle line is lost, I go looking for targets of opportunity and any surviving scouts. Even with this strategy, bad luck kills me quickly and often, but not quite as quickly as it had been. Also getting better at remembering to leave myself an escape route, since that's my only real defense, to duck behind something else.
I WAS looking forward (after a long grind) to getting to the T-34 and finally having a medium, but I've been considering getting some decent xp built up and then converted so I can grab a marder II with a decent gun. I've not heard anything bad about them.