I went from a 1.33 15" to a 1.67 16"- and gained the backlit keyboard. It's really cool, but a bit like the wavy effect when adding new widgets to dashboard- technology demo and not much else.
If you wasnt the 12", get it and be happy- while the 15" is more feature rich, it costs a lot more and is a lot heavier. I love my PB, but often wish for more portability.
And before the chorus of "wanna trade?" starts... no.