Who has the advantage: those enlisting with the pirates or those sticking with the existing militias?

With the release of Havoc and the continued emergence of Angel low-sec space (which has since disappeared) if will remain to be seen if people "switch sides" to farm LP on which side is winning. You can however see that no real "new" space is currently up with this link - https://sov.space/

It did seem at one point that "angels" as it were, were taking over a large area near the Heimatar border.