1

(9 replies, posted in SEO)

On reference of "more quality content" we can't achieve the rank on serp page since there are around 200 Google's ranking factors apart from content.
http://backlinko. com/google-ranking-factors

2

(9 replies, posted in SEO)

We have designed a separate page for forum that totally differ from its site and now if we talk about sub directory then that always has same design, lay out and structure so there is no way we choose later one.

I agree that sub directory is also good for SEO purpose now a day that is just because Google only show 2-3 sub domains page's result on SERP on the a single query according to relevancy but we don't have more than 3 sub domains (blog, seo, and forum)  that follows the Google's guide line.

3

(9 replies, posted in SEO)

The term quality of content plays a vital role in ranking, but i have never heard about having more content in sub domain or sub directory is also a ranking factor.

4

(9 replies, posted in SEO)

For blog or forum you should step up with a sub domain, the forum will be listed as a separate entity in the Google search results, which is good for owning the results and one’s reputation management. Sub domains do make it a little transparent for users which part of the site they're on(apart from bread crumbs).  for example compare 

forum.sagacademy.com

with

sagacademy.com/forum/