Subdirectories inherit authority
A blog at /blog benefits from the domain's existing standing. A blog at blog.example.com is treated as more separate and builds its reputation independently.
Subdomains isolate cleanly
A different application, stack or team is a good reason to separate. So is anything you want on its own certificate, cookie scope or server.
Decide once
Moving later means redirects, re-indexing and broken inbound links. The cost of this decision is low today and high in two years.
