I've also wondered about this difference. I've come to the conclusion that the larger screen will appear larger, even though it has the same field of view because you're sitting further away.
I think that our brain determines how big the screen is by how it compares to other objects in the room (if the room isn't perfectly black), and also by how the screen appears to move when we move our head. The further away you are, the less it appears to move when you move, so your brain determines that it is larger.
Clearly subjective differences, though. Technically, identical fields of view will give you the same picture, but the smaller one will be brighter (assuming the same screen gain).
Since a lot of movies are 1.85, not 1.78, I think I would prefer to chop off a bit of the 1.78 than have the less sharp "sliver" border when watching 1.85 movies. Of course, this may change as HDTV gets more popular.