Ah, that's the exact decision I had to make this past August. I went with the back pair of seats (middle of the three pairs), since I had experience with back pairs on NW
and loved it. UA
didn't disappoint either. The room, window views, and easier aisle/galley/lavatory access made for a fantastic ride. I like them so much that I'd be afraid to try anything else in a 747, especially if I had to pay extra like for Y+.
The space along the wall narrows as you go back. The first pair will have a surplus of room to store items since you'll have an extra seat ahead of you and the wall space alongside. The middle pair offers enough room to the side to store at least a briefcase size item--maybe a backpack. The last row is fairly close against the wall and won't offer too much more room than normal.
If you're a window watcher, back seats are great. It gives you a panoramic view if everybody keeps their shades open.
If you're one to lean against the window to sleep, the three pairs are not a great choice. Only the last row comes close enough, and even then you'll have to lean the side more than normal.
The back seats are a little noisier and move around more in turbulence. The video screen also is farther away. However to me those issues are minor compared to the benefits.