c4d works better with higher clock speed cores , this is why the iMac gave you better viewport performance , also sketch and toon render is faster with the iMac than mac pro to the same reason .
personally I would prefer the iMac than the mac pro , I don't need the extra render power , I can live with 2 machines for previews and small renders , and if I ever wanted to render a heavy scene or a big animation then I would send it to an online render farm .
google sync will support c4d soon , which I think is a cheap and good option (you can see IBC2016).