The simple answer: BOTH!

Lets say we have the plasma razorback turrets you can use the codex list just fine and those could be your HB versions. However if you're running Iron Hands then those plasma would be actual plasma in that list. Your hobby time and dollars are stretched and you get more bang for the buck! Basically the core of the force tends to be largely the dame, quantity and list structure being the differing factor. That means you can run a modular force where you can play 2,3 or even 4 different lists without much extra collecting /painting.
However on the chimeras I don't think any of the lists use the HB version but that's a rarer situation.
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