The MacBook Pro is very Popular because of the Power it has, the stability and of course the Design.
It has more input connections, the MacBook Air has not.
You can look that up on the Apple Supprot Pages, but here are some examples :
MacBook Pro has a Line-In for 3,5mm jacks, where the MacBook Air only has a 3,5mm headphone jack, but no input.
Also The MacBook Pro has a FireWire Connector ( the older ones ) which is great. MacBook Air "only" has a Thunderbolt Connector.
But if you have one Thunderbolt port you can probably plug anything into that because of the flexibility. But it is also very expensive.
I also like the handling of Music Interfaces in mac OS X. It is very easy to extend the system regarding audio-stuff that has USB. Most Macs "just work" with them.
Also you have a large Software Library for Professional Music Production. Including the free GarageBand package. And the Apple "Logic Studio"
Most people like the MacBook Pro because (I think) it is the easiest and most stable to work with on Music and Audio.
If you want to buy a MacBook, I would recommend either a low budget MacBook Pro 13" or 15", or the 13" MacBook Air. It's depending on what exactly you try to produce.
For most Musicians the 15" MacBook Pro is the way to go, because of it's great Power. You don't have to care about that any more.
The SOundCard inside the MacBook is pretty stable to hande a lot of tracks at the same time, but if you want a workstation (with many interfaces and mixers), have a look at existing MacPro setups from the past years (not the new trash-can macPro).
I would also recommend researching on YouTube for "room Tours" or "Setup Tours" of People who produce Music.