I think it was in the last keynote where Apple said the current iMac with fusion drive is the fastest machine for FCPX.
The MacPro will be given up or we will get a huge performance update to surpass the new iMacs. Both is possible.
You never go wrong with an iMac. It's also a perfect time to buy an iMac since the new ones are just out.
I'm still working on a late 2009. I wouldn't need 4 hours to export 14 minutes of a 720p video (of course this depends of the effects and filters you use).
The cheapest Mac Pro would not be very fast. The i7 iMac would definitely be faster but I would really recommend ordering additional ram to the iMac when it's purchased because it is very difficult to add it afterward, and FCP X would run much smoother with more than 8 gb.
If you get it with a fusion drive, just run your operating system on the fusion and not your events and projects. If I had to choose between extra ram and a fusion drive for fcp x performance, I would get the ram.