I'm a relatively recent Apple convert; I was looking at the 17" and 15" PBs; I went for the 15" in the end - the 17" was just too big. Otherwise, with the built-in Bluetooth and Airport Extreme, plus the bigger screen, it definitely had the technological lead. Just too gosh-darned big for me.
Your lap-size may vary. Also, I hear lots of rumours that Apple is going to announce a new 15" PB with an aluminium case (like the newer 12" and 17" PBs) at WWDC in June.