It actually depends on which page the link is related to. You will obviously be using keywords as anchor texts to link back to your site. If your specific post is about that keyword, then you must link to the specific post. That way, the user will get to read what he wants on your site straightaway instead of getting to the main page, then searching for the right post to read.
Also, linking to the specific post will also help that post rank in google for that keyword easier than the main page since the specific post is focused on that keyword.