i plan on buying a new MBA, probably the 13, the main motivation being as a music computer.
each machine represents different sacrifices.
13 MBP has low resolution (a dealbreaker for me) and is heavy (relatively), but is very upgrade-able, has or allows for (much) more storage, and i believe has better battery life.
the MBAs have less storage, but are so much more portable, are now comparably fast CPU-wise, and come with speedy SSDs. the storage space is less, so you may not be able to keep ALL your projects on your drive, but 256GB is a LOT of space for music production. the displays on the MBAs are MUCH better, which is huge for me personally. while the 11 is extremely tempting, i don't travel much, so i will probably go with the 13 as the screen real estate will be greatly appreciated.
... as far as the display goes, remember to consider the resolutions, not just the screen sizes. this is not just about crisp text and pretty pictures, each element in a program like garageband takes up a certain number of pixels (some things can be resized, others can't). so you can see a lot more of what you're working on on the 13 MBA than on the 13 MBP.
all three machines are great, so you can't go wrong. go in and play with them, opening up garageband on each. google screenshots of logic and ableton so you can see what those programs would look like.
by the way, i'm most likely going with 128GB. apps won't take that much space and projects aren't huge either, but when i have to dump to an external, no problem. for me, the jump in price is not worth it.
good luck! keep up posted.
