13" is plenty for music production
have been happily using the white macbook for 4 years, have switched to mbp 13" few weeks ago for more processing headroom
I get 28% at this stress test
http://forum.ableton.com/viewtopic.php?f=1&t=111880
with 8 tracks, and can go to between 20 and 32 tracks before hearing clicks depending on latency/sampling freq, still using this test,so 2 reverb per track, mid quality, + 4 aux with 2 verbs so that's between 48 and 72 reverbs working at the same time.
Unless working at high sampling rate you'll probably won't need this much processing power.
Plus easier to carry and good for gigs (where space is limited and it's easier to hide).
15" macbook are a waste of money for this usage.
You better upgrade the ram, add an ssd and an optibay (yourself it's cheaper)
The ssd works great in this context for samplers like kontakt that use direct from disk reading.
And if you need a graphic card just for gaming, the difference beetween 13 and 15" buys you a good windows desktop.
one issue though: the fan kicks in full speed when using live on a second monitor, due to the graphic card being on the same chip as the processor and working harder. So it's a bit noisy (triing to find a workaround right now)
It won't happen on internal display unless it's a heavy session, and no problem at all with protools.