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I would say this is easy to answer yourself - it depends what you use it for and how frequently you travel and whether you can afford it.

If you need a machine with lots of power, especially for CAD or graphics or photos, then 17". You will appreciate the screen size (especially high res). If its just for average use then 15" will meet your needs. They are about the same weight, the 17" size is getting to be a little awkward, but then the 15" cant be dropped into a pocket either. The cost - only you know if you can afford it.

I bought a 15" MBP for the mobility but in the end I found I travel so much and am away so much that its too small now - I need a desktop replacement and a high res 17" screen for muscle and graphics work (and DVD's). I got tired of waiting for a small powerful laptop from apple so I just got a 2nd hand 12" 1.5Ghz powerbook for short trips, holidays, email, light use.

I guess the 15" for me was neither big enough or small enough, meeting neither requirement.
 

I agree. It is hard finding the perfect laptop. I am still looking for that true desktop replacement that runs OS X. The 17" MBP does a good job, but it's no iMac or Mac Pro IMHO. If Apple made an 18" or 20" with all the bells and whistles of a true desktop replacement then I know I'd have my $4500 ready to smack down on the purchase counter.
 
after my 13in macbook, my new 15in (1t's 15.4in officially, no?) feels huge and beautiful. Why not get the 15in and use the change to buy a 20in cinema display?
 
It's not about being which is better or worse; they are the same computer, just 2 difference screen sizes; you want a more portability at the expense of less screen real estate--15"; like-wise, want more screen real estate at the expense of more bulk--17".
 
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