Subfolders in most instances for dat link juice, obviously, however both play a role depending on what technology or CMS needs to sit where.
You couldn’t put a self-hosted CDN on a subfolder and maybe you don’t have the knowledge, resource and capabilities to set-up a reverse proxy to put a blog on a subfolder, so it’s just easier to whack it on a subdomain.
The boring answer is that it depends…
…a better debate is honestly trailing slash or no trailing slash.
THE ANSWER IS NO TRAILING SLASH, BECAUSE A SLASH SHOULD DENOTE A FOLDER IMO.