Their is a very big difference between blog.abc.com and abc.com/blog. It means you are looking for blog in abc.com website or its a page of abc website. But on the other hand blog.abc.com is different. This is the main URL of website.
Hey buddy.. there is a lot of difference b/w these two blog.abc.com is a subdomain of abc website. abc.com/blog is the internal link of abc website or you can say a deep link.
hi blog.abc.com is the sub domain of your website, other is internal link of your website. In search engine point of view . when should u use the sub-domain? when you have different services then use the subdomain technique and when should u use the folder? when you wanna non product base website which is very small it is easy to build with folder. search engine gives more importance to folder base website