2) Username column on top of post is compatible with the current version of miniBB, it still allows to recognize post's content amongst usernames, and it is suitable for the users who pay attention at username before reading; that way they are reading from top to left to right. With this model also there is a big opportunity: each message takes its own table, and that's why the page loads up gradually. On slow connections you don't need to wait until the whole HTML will be loaded up. However this layout limits the using of additional info near user's nickname; to avoid a lot of empty space and unpleasant look of the page, you must care about small size avatars and limit the user's information provided in post.
I agree with that option and with vishal but there's no ideal solution for everyone. And that's miniBB's power, that can be easily changed to whatever a designer wants.
IMHO, putting user info at the top of a message is a great solution for everyone that integrates miniBB in it's own layout but have limited width. In my fixed width site i had only 480px available width and i couldn't waste one column for just user info so putting it at the top of each message (as a matter of fact i have put it inside message box) was the only way to leave all the available width for the message itself.
On the other hand, if miniBB is being used "alone" in a site which is mainly a forum site then the free space is "too much". Looking at this forum in a wide screen and with my browser opened in a 1024px window, i like the first page where a right column has thread's infos but reading messages in a thread (like this one) and replying, reveals me lot of empty (and wasted) space (like the one next to this message box) and also letters in every message seems a bit enlarged compared to what i usually see, not ideal for long messages because it destructs your attention from the message. So placing userinfo in a separate column would be better (for me).
But all these are just personal taste and can be easily changed according to everyone needs. The most important thing in a tempate is not the template itself but the easiness to change it!
PS: Most important thing for miniBB is to stay focus in "mini".