for the official system requirements of cc2015, they only list the K5000 and gtx 680 for desktop cards.
unless you need the 10bit output, or just want a quadro, you can use the gtx card and save money. i dont run mac, but i've seen others use the new gtx cards in a mac, i guess it only looses the boot screen? not sure if there is some other trick involved, but if you can do that the gtx 970 would be a good card to get. the new gtx 970 wouldn't work with AE ray-tracing, so if you are using that instead of C4D, you will have to use one of the older cards.
not sure if this is related, but someone posted this Beware: Software update June 30 2015 for Mountain Lion resulted in latest Nvidia driver 313.01.04f04 no longer valid for 10.8.5.
but i've seen others use the new gtx cards in a mac, i guess it only looses the boot screen? not sure if there is some other trick involved,
Dave finally put a website together for his side business of making PC-based NVidia cards boot properly in the older Mac Pros.
Dave's the guy that knows it all. He burns his own ROMs for the cards so that they boot with the grey Apple screen, and run at full PCI speeds. Talk to him, he'll steer you right. As for "which card", I'd stick with the newer 900-series cards. The Xeon(s) in your Mac Pro aren't going to be able to keep up with the 900-series GPU, but the important bit is that the 900s run at significantly lower power draw. And that's a huge limitation in the Mac Pro tower.
great info jason