my 15" macbook can be carried in a small messenger bag, but it's quite a strain on the back, so i mostly opt for a backpack anyway. a friend's 17" still fits in a slightly larger bag, so it's just as portable, if slightly heavier.
however, i could not do serious work on my 15" screen; not only is the fact that it's attached to the keyboard ergonomically challenging for me (because my hands and my head are quite a distance apart when in any comfortable position), but it's also too friggin' small. i've got friends that can work on small screens with ludicrous resolutions, but i'd prefer a physically larger screen to a higher resolution every day. so my tip would be on the 17"; for slightly better posture and prolonged health to our beloved devs! :)