Hi MacBook Air is more expensive than Pro generally but I went with the Air for playback personally because of the Solid State Drive - silent and almost instant.
As soon as you lift the lid the iTunes interface is there and music starts playing within 1 second. If totally shutdown it's the same but a few more seconds. Superb.
Biggest consideration for you is how much storage you will need - I bought a 13" Air with 128GB SSD and quickly needed to add storage - 128GB equals about 100GB when other system uses etc are taken into account.
Note that you cannot increase storage inside the Air due to the way it is constructed - if you think you will want 256GB you need to spec. it that way from the outset.
In the end I added a 256GB solid state external drive which is very small and discrete and again is totally silent and instant - cost about £400 though.
Final bit of advice is to buy a refurbished model from the Apple Online Store - mine looked as if it was brand new and I bought it for £920 or thereabouts - approx £130 saving on new price.
Pro is tempting however for storage space (think you can spec. it at 400GB storage but not solid state - new Pros may offer SSDs). If not already using a Mac for playback I would highly recommend buying BitPerfect for iTunes application which is about £7 and works seamlessly with iTunes and improves the sound quality with no negatives experienced at all.
I used a Toshiba laptop for playback previously and noticed some fan noise during quiet sequences, have never heard a peep from the Air. Brilliant laptop, great to use for other uses too.