Okay Okay, let me throw my cents here into the pan.
So if we're comparing AP, Zoro should be leading with his
57 Megatons compared to Sion who I asuumed to be stronger than
34.05 Megatons if she compared to Pray Master. Zoro has Asura, which can triple his strength, but if that is allow then he be at Mountain level, so push that to the side. He has another Stat Amps in the form of increasing his muscle mass, so he is technically more stronger than 57 Megatons. This is a disadvantage to Sion.
In terms of abilities. Sion's Forcefield Negation....idk how this works because technically Busoshoku Haki is a forcefield creation so maybe she can nullify it. So this is probably a disadvantage to Zoro if that is the case. Moving on, Zoro has a resistance to Gravity Manipulation so that will help him against Sion's Gravity Dance. With Observation Haki, Zoro has the potential to be able to handle a Danmaku attack.
Sion is an accomplished Sword Saint and with her Power Mimicry, she can copy Zoro's fighting styles and by which, I believe that she can use that to predict his movements. But Zoro can counter with Observation Haki, but it will still be tough.
A slight disadvantage for Sion I see is that of the range. Since Zoro has the biggest range with hundred of meters compared to that of Sion's best range of tens of meters, they will start at Zoro's range. Where he can launch attacks at her from a great distance. Though Sion could counter Zoro's Air Manipulation attacks with her Forcefield Creation and force a direction confrontation or use her Danmaku to disoriented her opponent. However, she has to be careful since Zoro is packing a lot of strength and he has shown to be able shrug off attacks from those lower than him with utter ease.
I don't know how Pray Style will interact with Zoro's Three Sword Style, but I leave this for other who know the verse to answer for me since I don't know much about Pray Style.
I vote for Master fisher (that is Zoro I assume) for now.
EDIT: Zoro doesn' have Reistance to Gravity Manipulation but he has shown to be able to somewhat move under unknown amount of gravity pressure -
https://imgur.com/a/xDWdy, but this is coming from a guy who can exert enough gravity manipulation to bring down a meteor).