“Subdomain vs Subfolder” for SEO is actually a very hotly debated topic on the internet. Google it and you can read for days. You can even find google officially contradicting itself.
One side of the debate suggests that subfolders are better to ensure that all of a sites contents count together for the search rankings. The other side of the debate says that google no longer treats blog.example.com and forum.example.com as two separate sites. Folks in the middle throw in some “facts” suggesting “popular” “big” sites like homedepot.com or walmart.com are treated differently than Joe’s blog and forum, and there may be a traffic threshold. Still others, suggest it’s how the sites are configured within google’s webmaster tools and how well they link each other as to whether two subdomains are related. Still others proclaim 2 subdomains will be treated as a separate site unless the google bot finds a “site map” tying them together.
The jury appears to be out, but if SEO is what’s important my choice would be the subfolder format. If I were just picking what I like I would go for subdomains.