When person account are activated on a Salesforce org connected to SFMC with MC Connect, what are the best practices regarding the Subscriber key and Contact key ? Should we use the (person) account ID or the contact ID as Subscriber key AND Contact key in SFMC? (Especially, when creating a Sendable Data Extension, we have to define the Send Relationship.) Or does SFMC automatically use one of these ID as subscriber key AND contact key ?
For Orgs with PersonAccounts enabled, you still will be using PersonContactID as Subscriber Key. There is more info on Subscriber Keys in a multi cloud scenario in this article.
It can be difficult to find the Contact ID in a setup where PersonAccounts are enabled, as the contact redirects to the Account page. For this purpose I use the Salesforce Inspector Chrome plugin, which is able to display all the fields on a PersonAccount page, including Contact ID.