Help with 2 Apple IDs and Apple One

Hello,

I have two Apple IDs because I started all of this in the .Mac days. One ID is an iCloud email address and the other is with a third party email address. All of my old iTunes music and movie purchases were made with the ID with the third party email address. I'm interested in signing up for Apple One and would like help with how to set this up properly using these two Apple IDs. 

Is it better to use: 

1. The iCloud email ID as my main Apple ID (the one I would sign into when setting up a new Mac or iPhone) and then sign into the third party email ID under "Media & Purchases" within the Apple ID in Settings on iPhone or Mac? OR
2. The third party ID as my main Apple ID and then sign into iCloud within the Apple ID in Settings  on iPhone or Mac? I think this is possible, but I do see an iCloud section here. 

Thank you!