Just want to throw this out there . Windows 10 for Workstation is slightly different beast that supports a couple extra things like resilient file-system.
Microsoft for whatever reason charges more to OEM's for computers with 4 or less cores trying to use this version of Windows 10. Why I don't know.
I don't think there is an actual difference between the two Windows 10 for Workstation versions and seems to just be license difference and any customized machine from an OEM is just going to force you to change to Windows 10 for Workstation for up to 4 cores, if you select that an a 4 core I7 or Xeon. But if you upgrade to a 20 core CPU later, that install should support it and I don't believe you break the licensing (since 4 cores or lower is an increased priced feature).
The one thing I can figure is they want a little extra money from people who are trying to do a barebones File server with the reFS instead of using Server 2016.
Yeah it's an odd situation for sure.
IMHO :
*IF* Dell is offering W10ProWS at no additional cost with the package. Take it and run, you're gold. If you should wish to upgrade to a 4 socket setup later, you are already ready to go.
*IF* Dell is wanting to charge extra for it, and you want to use W10 Pro, W10 Enterprise, or W10 Education, fine. Just use the drivers on their website, and you will be good with either 1 or 2 Xeon 6130s, and can upgrade to any other 1 or 2 CPU config in the future up to 256 total cores (so a hypothetical dual 128-core Xeon setup would work, should Intel release something in that socket, however unlikely or likely). If it happens that you later want to fill all 4 sockets, then at that time you will have to pony up for the W10WS license @ $309ish, or simply change operating systems to something else that does support 4 sockets.