Please, keep in mind that it is just a mere CONCEPT of our GUI. Many things might change in it. Do not judge every pixel in it, please.
(Click for full size):
Really love this UI, the only thing that I believe could use a tad more work would be the health and stamina bars, something a tad more complex and not as large like the other UI.
Taking into account the full design shown in these screenshots (not just the flatness aspect), I much prefer this GUI to the other one. I hope there's a toggle, though.
The volumetric one looks more stylish and professional to me than does the flat one. I think the flat version has too much of an angular zig zaggy look. Particularly with the smaller lists where it is, for me, harder to make the necessary visual separation between the adjacent lists because their frames "jigsaw" together like that. I like the fact that the volumetric one has much better framing of the option lists. Generally square boxes, or rounded square boxes, with fancyish borders are better...more in line with UI standards found in all software interfaces, and therefore more familiar.
I prefer the volumetric one because it is more beautiful and attractive. I also think it expresses the medieval theme more effectively...like signage commonly used to represent that era.
I think the modern feel of this GUI contrasts the game quite well. I especially like the color of it and how it shows up better with the game in the back ground. As long as we can still move the HUD around to where we want it this is a great look. You have my vote.