I returned from a week-long photography trip recently. On such trips, I do a modest amount of image review, I take care of customer emails and subscriptions, etc— typically at the end of a 14-hour day in a remote area (no hotel or restaurant!), so I have little patience for a slow computer.
My needs are not those of everyone, but here I share my experience with three Apple laptops, all of mid or late 2011 generation, and all with SSD drives.
MacBook Air — too slow
I eliminated the 1.7GHz 13" MacBook Air as a candidate for my field work after my last trip; it was noticeably sluggish for every important task I perform, making it a irritating device when I wanted to get things done and get a night’s sleep. The same operations on the 13" MacBook Pro were considerably faster.
Also, the lack of a Firewire 800 port on the MBA means waiting 3X as long to download a Compact Flash camera card over USB, and 3X as long to make a backup over USB. That said, I bought several FW800 readers when Lexar and SanDisk discontinued theirs, and it now appears that none are available from any vendor, so this point is a non-option for users without one. However, the 17" model does afford the option of an ExpressCard/34 reader.
Based on experience, I now consider the MBA a usable but mainly stylish computer mostly suitable for web and email and other casual uses. I do like it, but it’s just not up to speed for the work I need to do. Because I travel by car for photography (not airplane), the light weight is not at an advantage for me, but might be for many users. Perhaps the next generation model will resolve the speed issue.
MacBook Pro 13" — fast
I tried the 13" 2.7GHz MacBook Pro on the next trip (and simultaneously with the MBA on the previous one), and it was a big improvement in performance over the MacBook Air— the sluggish behavior was gone. Side-by-side, the speed difference was immediately obvious.
In terms of performance for my work, I’d rate the 13" MacBook Pro as an excellent value, and very close to the 4-core 17" model. The small screen is its only real limitation.
MacBook Pro 17" — fast and a large screen
On my most recent trip, I utilized the very latest 17" 2.5GHz MacBook Pro— the fastest MacBook Pro available. The 4 CPU cores of the 17" model helped a little over the dual-core 13" model when processing images, but in general performance was hardly to be distinguished from the 13" MacBook Pro. So on a performance basis for most tasks, the 17" has little to offer over the 13" (CPU cores are frequently underutilized, so 2 cores is usually plenty for most tasks, and the 13" model runs at a faster 2.7 GHz).
What sealed the deal with the 17" model? The large screen. This is an efficiency issue for me in multiple ways: image evaluation, editing of web pages, and even reading emails. More in view at one time means less fiddly scrolling around, and easier image evaluation.