The MBA is cheaper, lighter, thinner, and faster at anything that's disk latency bound (such as startup and launching apps).
But if you need to use DVDs, CDs, firewire devices, plug-in ethernet, or require more than a few ten's of GB of storage, all without carry any accessories, then the MBA a no-go. The MBA is also not the fastest at tasks that require minutes of CPU crunching, nor does the MBA 11 have the longest battery life.
Depends. If you're talking about anything other than the Ultimate MBA, then yeah. But the Ultimate 13" MBA is faster than a 13" MBP in many ways.
I gave up my 13" MBP strictly for a 13" MBA Ultimate. The 13" MBP is 2.4 ghz (i guess you can pay $300 extra for the 2.66 though), 4g, 250gb HDD. The MBA ultimate 13" is 2.13 ghz, 4gb and 256 SSD. Having had both, the MBA and the MBP processor's speed differences are not noticeable. The ram is the same. The one thing that IS noticeable is the SSD. The SSD changes the entire computer and if we're comparing a stock MBP 13" to a stock MBA 13" Ultimate, the MBA is faster because of the SSD.
I did put in an aftermarket SSD into the MBP so that can be another route. If you do that, depending on the SSD, the loadup and bootup speeds will be equal, slower or faster by fractions of seconds so it would be equal.
Basically a MBP with a 256gb SSD would cost nearly the same or just a little less than the Ultimate 13" MBA, making them "equal" in terms of performance to cost ratio.
Then, its just a matter of features. MBA is lighter, thinner, portable, screen is hi-res at 1440X800, battery life is more efficient. MBP (with a SSD) would be heaver, bulkier, lower screen res at a mere 1280X800, but has an optical drive, backlit keyboard, slightly better colors on the screen.
It was worth it to me to give up the backlit keys and optical drive and the firewire, (all of which i dont use) for portability.
Battery life on the MBP is worse than the Ultimate MBP but in my experience, a MBP with a low power draw SSD will be equal or just as good as the MBA. I had a Corsair with .5w idle and 1.5w active and the SSD alone extended my battery life by A LOT.