There is a nice review by Vincent Laforet, Does size really matter? Debating the 12 - Inch vs. 15 - Inch PowerBook.
In addition to his points, do I need to carry a large screen around? There's comes the considerations.
Basically, PowerBook 15 inch provides better (brighter) display in addition to work space. the ratio is 5:3.
The PowerBook 12 has standard screen ratio 4:3, slightly dimmer, but comes with all the gadgets except the gigabit ethernet, FW800, PCMCIA card slot, smaller maximum RAM size and less memory in video card.
For real professionals, they carry 17" PowerBook with external FireWire drives. They use 22 megapixel digital cameras. (Steve Kalalian - Digital Imagiing on Steroids)
For film makers, they definitely need 15" or 17" PowerBooks.
For rest of us, a laptop should be easy to carry. PowerBook 12" is still the most compact, full feature laptop notebook around. Unless you are able to carry the 17" around, or you want to use it as desktop replacement... I would opt out that option. Between 15" and 12", it's a bit harder to decide, when the weight is not much lighter in 12" (compared to 15").
My recommendation, unless you really need to work in larger screen. Go for 12", make your life easier and you will sure be able to carry it around a lot easier. It's a laptop, make it portable!
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