Let say I have a tech site and I am writing a post and mentioning "iPhone". I have previous posts about "iPhone", should I link to a post or to a page with the "iPhone" tag?
I think the link given below is the good for internal linking.
Most blog software will do both for you. Your post link should be traditional in that the link is the title of the post with a snippet of the the content. If that is the case, users expect to go to that page and read your post. Do not disappoint them. Each post should have tag links that if a user wants to read more of your posts on a particular topic, then they would click on the tag link.
This is the traditional way and users expect that. The general rule with websites is not to make a user click more than once unless they are using some sort of navigation which is exactly what a tag link is. Take them to your post if that is what they are expecting. Do not disappoint a user by making them click twice.