Bottom line, all of the advantages/disadvantages of one vs. the other comes down to a matter of preference and the specifc in-flight experience. All of the differences are "on the margain", IMHO.
If you have a wide girth, you'll appreciate the extra width of the 777 seat. If you are rather tall, the extra legroom (about 2") may mean a lot to you on the 767. If you like peace and quiet and a sense of space, the 777's cabin will suit you. Want some extra room and solitude, go for the middile aisle seat on the 777 and pray for a light flight and no one will sit next to you. Hate the IFE that is canned, fly the 767 and get your own personal video player on Int'l (not Hawaii) flights.
But, in the end, if the food and/or the crew are not good (it happens sometimes) and that is important to you it will not matter what plane you are on.
------------------
What do you want to be, American?