The real advantage with 50mm is fisrt its much mor versitlie unit and it cant be decrewd by mortar rounds, this works both ways ofc beeing ince the vec is dead you cant reuse it.
you normally decrew pak gun by using small arms, grenade, or mortar, all of which are much less effective against Ht. The same thing that will destroy a ht will also destroy a pak as well.
You can also repair the HT.
And ambush bonus covers what? Accuracy(1.25 acc) Pen (5x Pen) and Damage (1.25 dmg).
Firing faster equals more DPS, and more range means you're just flat out better ;p.
So, like I've said - pak gun is in every way superior to the 50mm's gun.
Even if the pak gun is superior simply due to the awesomeness of ambush, the 50mm at ht still carry a very potent gun.
From what I've seen, the 57mm doesn't act as a direct hard counter to the 50mm atht. Normally the 57mm will win, but that's mostly due to the 57mm's much better DPS. It's like sending a 76mmto kill a pziv. You usually need at least two 50mm atht to kill a 57mm.