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So, what is your take?

BS4 vs PE?
Against non-vehicule, either you fire with Blast (obviously) or with weapon that need to "roll to hit", PE is better.

For weapon that need to "roll to hit", it's better by a very slight margin: only 1.3%

PE is 49/72 x "To Wound roll" = 68% x "To Wound to Hit"

(1/2+1/6*1/2)*(1+1/6)*"To Wound roll"

BS4 is 48/72 x "To Wound roll" = 66.6% x "To Wound to Hit"

(2/3)*"To Wound roll"

For blast, it's really better: 16.6%

PE is 7/6 x "To Wound roll" = 116% x "To Wound to Hit"

BS4 is 6/6 x "To Wound roll" = 100% x "To Wound to Hit"

Only against vehicule, BS4 would be better, as PE has no effect.
But as your Vanquisher will be your Tank Commander, that won't be a problem.

The "Tax":
Hydra is almost always useful, especially against top tier army.
But as you can only take one, that might not be enough.

The Enginseer is good for PotMS (that will help you to fire a weapon without snapshoting on one of your Ordnance LR) and might sometime repair one of your Hull Point

Defense vs Offense:
So, in the end, if you take lot of LR with Blast and have a way to get more "HIT"s (Prescience), the Steel Host might be better.
But for Punisher, Vainquisher and Exterminator in the end, the little offense increase is negligible compared to the Good Defense Bonus and you might prefer to get an Emperor's Fist.

Remember also that Pask can give PE to your LRs in the Tank Commander Squadron.