Just from the top of my head. I might have missed something.
One code:
- accurate no. of unique visitors
- the need to use segments to get data for specific subdomain. Also additional setup needed with custom dimensions to be able to segment the data. Not all types of events have event_url set.
- tricky setup when it comes to integrations with gads and search console, since one property can be currently integrated with one account
- full customer journey reporting. Especially nice if you have a marketing website and some post login area. Being able to merge that data into one makes it possible to generate valuable insights. E.g. the ability to see what marketing channels brought users that converted later on within the product.
- sharing one consent popup between subdomains
Meta sites:
- no visitors deduplication
- data is clean - one property per subdomain
- straightforward integration with gads and search console
- customer journey aggregated per property
- new consent popup per subdomain
I usually set up both at the same time, but it makes the setup a bit more complex, since custom codes are needed to have 2 analytics trackers on one website.