Fares aside, since they're not consistently better or worse - the Porter PY experience is subpar compared to the Westjet Premium experience when it has the proper cabin, and about equal to the Westjet Premium experience when it has the transformation cabin.
I guess you could book Porter PY simply out of spite, but there's still a roughly 85% chance of having a better experience on Westjet.
It doesn't matter if you "know exactly what you're getting" what that's always consistently equal to or worse than the alternative.