I personally would wait to see how the new Mini is equipped. The Mac Pro would more than likely outpace the mini for doing video rendering, if it has dual quad-core or dual hex-cores. But for most everything else, the Mini should be faster.
Although Intel's knowledge base shows that even the current gen Mini's CPU should be able to handle 32GB of ram, I have not seen anyone try to put two 16GB modules in their Mini.
I'm hoping that a new Mini, if it arrives, would have a CPU close to somewhere close to the late 2013 Macbook Pro 15". If it does, then it should be fast enough to do just about anything, except video work such as Final Cut Pro and others, and be more affordable than a 2013 Mac Pro.
But back to original comparison, the 2009 Mac Pro seems to be a good value at $650, because even the current Mac Mini, once you upgrade the memory and drives to SSD, will be a few hundred dollars more and may not give a better video experience than the 5770. I think the Iris or Iris Pro would yield closer results to the 5770.
However, if and when Apple does release a Broadwell based Mac Mini, the graphics should be even faster. According to Intel, about 40% faster than Iris. So throw that in the mix and then weigh all of your options.
Me, I'm getting the Mac Mini that is hopefully released later this year.