IMO it is more about ease of use for the end user and less about SEO. If you have a good help sub-domain, it will automatically redirect users to the product site.

Still If I had to make a decision, I would have compared metrics like pages/visit, time on site, bounce rate of help site and main site. If help metrics are better than the main site, adding content to main will add value else it will deplete value. Also if it is only one product, it makes sense to have help within main site bur for multiple products, you should be better off with sub domains (product wise and not docs vs main).

Please see and decide what is best for your users first, keeping SEO at second priority. Hope it helps.

• Nitin