The iMac 5K is going back. I love the screen, but in the end a full-spec iMac is around $4K + $400 for memory plus tax plus AppleCare plus external storage—we’re talking a $5K machine, at least. In that territory, the equation changes, at least for me.
Too much and not enough: a 4-core Mac Pro is a solid long term investment with 6 Thunderbolt ports on 3 busses, faster SSD, an upgradeable CPU, along with 2X or 4X the maximum memory. And the display is independent and can be truly calibrated.
The iMac 5K is just not for me, and I’d rather have my $4K+ back—the 2013 Mac Pro is my workhorse and the screen on the iMac is just not as big a plus as I thought. It’s very nice for viewing, but not really a good match for critical color work, and I’d rather have the NEC PA322UHD on my Mac Pro anyway (pro-grade with 1 delta-E color calibration).