Design-wise, I like the use of metal but I am not a fan of the belly in the back. I don't know if that's technical (needs to cram stuff up while looking thin) or ergonomic consideration, but I was never a fan of those designs. Phones or otherwise. I dislike those laptops that look thin from the fronts but have fat asses.
That was always what made Apple designs attractive to me, regardless of specs. And currently I like my Nexus 4 very much, and also like Xperia Z's design. Neither has those convex bellies.
As for materials, my preference goes to: Glass > Metal > Plastic. So even if I were given a choice now between Nexus 4 / Optimus G and HTC One, my choice would be the former. (Though I have to admit that the new camera makes me very curious)
Hopefully Android OEMs will get a cue and come up with a flat tablet. That's when I will upgrade my Nexus 7.