Resolution is the only thing I care about. My 14" T61P w/ a 1680 x 1050 pixels is great and will show all the same details that a 17" model with the same resolution will.
I can't stand the lack of room on Traktor with anything smaller than 1050 horizontal pixels. I regularly used a 17" laptop for a while, and I'll be honest - go with the 15" machine with a good screen.
Bookmarks