Regarding the Air, I've been running one (actually on my third now) since 2011. All of them have been the i5 model and have all been up to the task of Traktor and relatively basic Ableton Suite and Maschine use. I started with the 13" for about 2 years, then wanted something smaller so picked up an 11" with 1.6GHz i5, it was the last of the USB2 like my 13" so I knew I wouldn't have any of the USB3 issues that were happening at the time. Even though the 11" had a slightly slower processor I certainly didn't notice the difference.

Due to my line of work I've had the ability to try Traktor on a number of MacBook Pro's over the last 4 years, and purely for running Traktor the Air actually runs better than any of the Retina models when comparing how much CPU the Traktor process is using with the exact same OS version, Traktor version, settings, drivers and track list. The Air ran about 10% less CPU for the Traktor process, be it against a 2012 quad core 15" MacBook Pro Retina or the 2013 dual core 13" MacBook Pro Retina, and after everything is said and done I'd say the reason is the retina screen. Traktor is still not optimised for Retina and I think that makes a reasonable and measurable difference.

I've actually just picked up a very good condition 2013 MacBook Air 11" i5 with 256GB SSD for about US$440 in the last 2 weeks, and once again it runs extremely well. One of the guys I work with has an i7 MacBook Air 11" from last year and he edits multi-cam weddings and 40 min doco's with ease on it including fairly intensive plugins, and that's with footage from his Sony A7S cameras. Oh yeah, he also moved up to the Air from a similar MacBook Pro to what you have, and he went through the whole SSD thing with that and essentially its chalk and cheese the difference the Core-i series processors make, yes the SSD will make,the OS feel snappier but when it comes down to it your processor can't do more than its already doing, hell even my 11" from 2011 has a more powerful processor than your C2D.

Anyhow, don't count out the Air, the 13" with i7 upgrade, 8GB of RAM and 256BG SSD would be a very decent way to go. It has as much grunt as the 2.6GHz 13" Retina, will cost you US$50 less, it's lighter to carry around and it has 1440x900 resolution compared to the 1280x800 Retina (that's the res Traktor uses at default retina resolution, you can choose 1440x900 or 1680x1050 but things get blurry). The GPU actually runs quite well too because it's not having to push out the retina number of pixels.