Ben Lovejoy, on the upcoming Mac Pro:

So yes, Apple has promised that it will be the best machine in the line-up, meaning that by definition it has to have a higher spec than the iMac Pro. But perhaps not massively so. What you’ll really be buying with the Mac Pro is two things.

First, configuration options. Four USB-C ports is good, but as the standard takes off, there will be pros who’ll want more – and you’ll be able to add them. Up to 4TB fast SSD is very nice, but again, AV pros will want more, and they’ll be able to add it without having to hang them off the outside of the machine. And so forth. Unlike the iMac, you won’t be buying a sealed unit.

Second, longevity. The iMac Pro will be a stonking spec when it launches, but of course technology never stands still. A blindingly-fast machine today will look distinctly pedestrian a few years down the line. With a modular format, you should be able to simply slot in new components as they become available – including, I’m sure, CPU and GPU.