DELL XPS 8920 = Which MACPRO

The new 2018 Mini would essentially be a speed match if you get it with an i7 CPU and 16 GB of RAM.

It's difficult to compare the current Mac Pro to any PC, or even other current Mac models. The design hasn't changed since 2013.

Here's a speed test comparison of Mac models. As you can see, even the newer laptop models trounce the current Mac Pro. Maybe a bit surprising, the latest i7 iMac beats even the iMac Pro.

But those are single core results. Click on Multi-Core button and the list changes dramatically. The iMac Pro then trounces everything. Even the 2013 Mac Pro comes in just ahead of the 2018 Mini.

For intensive video work, the 2018 Mini won't exactly scream through the work, but it will do. Though you can pick up used or refurbished Mac Pros for not much more than the same price as a Mini configured as your comparison. You can find similar modes for even less on eBay.

Your question is limited to compute power. That issue is complex enough, but is not the entire answer by any means.

Overall performance for a particular kind of work may be more dependent on I/O ability, drive organization, or particular graphics features or graphics horsepower. Also, Exporting to certain Video formats or using Apple-TV is MUCH faster on a Mac that has Intel Integrated Graphics processor available (because it has custom instructions for these tasks)..

So to make reasonable comparisons, you would need to include what sort of work you anticipate doing on this computer.