The quad core 2.6GHz 2012 model is still the flagship model. That 2014 dual core model should still be a great choice for audio though. Perhaps even the same performance with audio even with half the CPU cores. The advice is still to shop for speed vs. cores for audio work.
It has a slightly faster idle speed but the boost speed is actually a little higher on the 2012 model. But then the 2014 model has the M.2 pci connected SSD. But then the 2012 model has 2 2.5" SATA drive bays.
Those fusion drives are a poor choice for audio. Good choice to not go with that! 2 drive bays in the 2012 model is nice but the pci connected SSD internally and then adding TB external drives would be even better performance. (Costs 4x of course.)
Sounds like a good choice for an audio machine and I'm actually not sure how much more bang for the buck spending more on a 2012 model would have been in this scenario.