How similar are the sites regarding their structure, especially their navigation structure?

For the recommended Language First Hierarchy the navigation structure should be similar throughout the hierarchy.

How many languages are spoken in the various countries I want to deliver my website to?

The more often a given language is chosen throughout the site hierarchy, the more the Language First Hierarchy will fit your needs.

Are there different external data references (such as product IDs) which differ per country?

If these references are strongly related to the country, you may be better off with the Country First Hierarchy. Otherwise, given the example above of the Swiss sites, you would have to manually synchronize external references in the Language First Hierarchy between German (Switzerland) and French (Switzerland).

What is the ratio of textual changes versus structure (e.g., navigation structure) changes you are expecting?

The more you update textual content, the more translation tasks will occur, and the more the Language First Hierarchy will fit your needs.

What are the costs of structure (navigation) changes versus translations?

The lower the cost for translation are, the less important it is which hierarchy you are choosing. The Language First Hierarchy pays off as soon as translation costs reach a considerable amount.

What is the structure of my editorial team?

If your editorial team is spread across various countries, each responsible for a given country, the Country First Hierarchy may be the better fit, as you will reduce dependencies between editorial teams.

If your editorial team is tailored by languages, the Language First Hierarchy is obviously the better fit.