Syncing messages when 1 device has more data than 2nd device

I use Messages on both my Macbook Air and my iPhone. Both are supposed to be syncing to iCloud using the same iCloud account; both are set to keep messages "forever".

I don't know how this happened, but: my Macbook Air Messages app has a far more extensive history of Messages conversations than my iPhone. I want to preserve those. My question: if I go tell my Macbook Air Messages app to "sync now" (navigate to iCloud+, select Messages, select "sync now") will iCloud+

1 -- restore all the messages from my Macbook Air Messages app to iCloud that do not appear on my iPhone, so that the iPhone Messages app matches my Macbook Air Messages app

OR

2 -- delete all the messages from my Macbook Air Messages app that do not appear on my iPhone, so that my Macbook Air Messages app matches the iPhone Messages app.

Essentially, this is a question about syncing: does iCloud add deleted messages during syncing, or delete added messages during syncing?