Best bang in SH Tanks is the Cobra II, although the Scorpion is also great for it has a multitude of weapons to use.
Cobra has a bigger gun than the scorpion and is a multiwound killer.
Scorpion has a more define anti infantry role.
This is how you would think it is based on the fluff and the models.
By the current rules, though, there is almost no target the Cobra would be better against than the Scorpion while having much less range and no anti-infantry capabilities.
It is cheaper in consequence, even more so since CA 2017 (or rather the Scorp more expensive), but I'd still take a tank that is actually useful over one that is not es expensive but still quite.