tim100,
funny you should say that. i actually just returned a 13" mbp a few days ago. i had a big problem getting beach balls that would last for 30 seconds a few times every hour. this is a problem affecting other 13" mbp owners and if you want the full story, you can find it if you search under my other posts. the problem is also discussed on apple's discussion boards.
but, that certainly does not mean you'd have the same problem too. truth is the 13" mbp is a great computer (if you have no issues with it). for me personally, i decided i won't be using my laptop for anything more than basic use (web, email, itunes, movies, word). with that established, i wanted something super light and the easiest thing to carry around when i leave my home. also, i believe the current line of mbp's are more glossy than the macbook air because it has an edge to edge piece of glass as a display. there is also another post on mac rumors where a mba owner has photo proof that the mba's are less prone to glare. i'll report back after i use my new mba a bit to see if that is indeed true.