Thank you for your inputs so far!
So I made the following calculations:
Since both hit on 2+ or 3+ respectifly after cloudstriking (good input), I will just think about the first round (hit on 3+) but both tanks behave the same so it doesnt really matter fpr wounds inflicted per point. The Cobra will average 4,5 shots with a reroll the scorpion 14 without reroll.
Cobra will wound on 2s, while the scorpion will wound on 3s mostly. So on average the cobra does 17,5 damage and 1 mortal wound. When taking into acount that the target has a 4+ invul it averages to 19,5 wounds before invuls. The scorpion does 23,33 when taking the 6 dmg on wound rolls of 6+ into acount.
This goes for 0,033 wounds inflected per point for the scorpion (0,73/PL), the cobra achieves 0,035 (0,70/PL). So quite similar.
SO it is basically the same and I will go with the one that I like more cosmetically, I will need to think about this for some time because its quite a big investment.
Just for interest: I also did the calculation for the revenant titan, just the pulsars do about 34 wounds efore invuls. So nowhere near the pair of scorpion + cobra would do (on power level gaming he is the same as both of them, on point game chapter aproved really stomped him into ground). Also he starts to degrade after 5 wounds taken...and the tanks could use Iyanden while the revenant can't due to <conclave> instead of <craftworld>.