For me the deciding factor was price - I just couldn't justify the steep premium for 17".

However, in retrospect I think it would have been worth the $ because the 13" screen is rather small compared with the large monitors that I use at work, and feels even smaller thanks to the widescreen format.

On the plus side, the 13" is far better for travel, it easily fits inside briefcases, knapsacks, etc.

In an ideal world (from Apple's perspective, too) you could buy one for home and one for travel.