Here is a good comparison of both in the same livery, the 767 tail bows a little bit down against the 330, but both are nice Aircraft.
The A330 I like most in Sabena and the B767 most in American liveries.
I like the A330-200 and -300. Also like B767-200. The B767-300 is acceptable but, sorry pal, I think B767-400 is ridiculous although this doesn´t matter for airlines when they are going to buy aircraft, I think. If it is profitable and fits well on certain routes, why not a B767-400??
Between a B767-200 and a A330 (no matter the version) I'd choose the A330 for best looking.
I like the 767 In US Airways livery as well as their livery in on the A330. It'll probably look much better with NWA colors regarding the A330. With the 777 UAL is pretty sharp on the 777. I hate the Delatflot scheme because it's too plain. If they added a cheatline then it would look alot better. The AA stainless steel looks great on any given day. Hope they do it with the 767-400 if they plan to get it. CO is a bit too boring on the fuselage.
My opinion is as follows
767's - Lan Chile
Canadian old old colours and the Goose scheme
330's - U.S Airways
Prefer the 330-200 but the U.S Airways 333 is still the pick for 330's.
I'll toss in the ugliest 767 - the United Star Alliance beast, the ugliest in the sky !!!