Sub directory will be best in my opinion.
In my opinion ,Both are best
It depends on requirement. For low priority or different section of content: subdomain. For SEO, Subdirectory.
Both serves the similar purpose but when comes to SEO you will have only two options.
Either it helps to pump your ranking or entirely puncture your business.
Sub-directory is easy to maintain and helps to gain overall domain authority but on the opposite side, you will face entire negative effect. If one sub-domain is penalized, this will affect the entire site.