Justification: Stats are important, and should be above the fold. We should not have both model and portrait above the stats. (User:Hefaistus/Sandbox1 also conforms to this.) IMHO, portrait is better to have on top because importantly (1) I think it is more recognizable and less importantly (2) it is smaller.
People see hero portraits far more often than they see full-body models. Hero portraits are seen during every pick phase and every CM match, every score screen, ten (or eleven) of them are permanently visible in every single game of Dota, they are the primary way of visually identifying heroes in the rest of the Dota 2 GUI (such as most successful heroes), they will be seen on the new card system, and they are found as user avatars in forums like Reddit. Some heroes are not immediately recognizable from their in-game models. Tinker is in fact an excellent example. I suspect a new player would have to look twice to see that Tinker's portrait is the same dude as Tinker's model. The portrait is almost all face; the model shows barely any face, and what face there is is angled. (Interestingly, it works the other way around as well: the model of Tinker from this perspective is not immediately recognizable as the same dude as Tinker from the in-game perspective.)
The most-recognized image is the one that should be used prominently on the page. Many (virtually all?) of our users view hero pages because they want to learn about that hero. The portrait is a critical piece of information to learn.
This would be less important if, as ChocolateWaffle suggests, "you get to the hero page it's because you already clicked the avatar in the nav". However, I suspect that statement is untrue; it is certainly untrue for me. I would guess many users come from Google, from internal links, from external links, and from the search box.
I have also included Nicko's guidebox for reference, because I think we should be aware of how much sidebar-height we might soon be using.
Jumpin' Jehoshaphat, that turned into an essay. I really don't care about this as strongly as my wordcount suggests. :-p