You can use an extra display with the iMac, and performance wise it's pretty competitive.
Some initial benchmarks of MP v iMac are here: http://www.anandtech.com/show/7603/mac-pro-review-late-2013/7
But for Aperture the story is unlikely to be as depressing as it is for Lightroom.
Apeture 3.5 is (apparently) optimised for the new Mac Pro ( http://appleinsider.com/articles/13/10/23/updates-to-aperture-final-cut-pro-will -tap-into-horsepower-of-new-mac-pro )and also Aperture makes use of the GPU for all processing, so I think for Aperture you will be able to exploit a lot more of the power than Lightroom can.
I'd advise keeping an eye out for Aperture performance benchmarks on the new Mac Pro before deciding.
Edit: Here's an interview with the photographer who got to use the pre-production Mac Pro: