I run both at my shop and they each have distinct advantages (the 5K Retina screen is gorgeous). It's always a trade-off between speed and cost. The faster you want to go, the more money it's going to cost. For many of today's task an iMac is a great machine. But if you need the horsepower for transcoding you're not going to beat a Mac Pro.
excerpts, quips, and musings on the production and post-production industry, and other stuff of interest