Not at all. Add a 10GbE card to the Dell for $350. A quick search for T-Bolt->10GbE found one for $1200 (
https://www.attotech.com/products/t...hunderlink/thunderbolt-to-10gbe/TLNS-2101-D00) - which is patently absurd. Please find one that doesn't cost more than the base price of the Dell. (I pay about $350 for dual-port server adaptors that support 10GbE, 8Gbps FC, 10 Gbps FCOE and iSCSI with full offloading engines for all four protocols. I can't believe that the "T-Bolt tax" is that high.)
If you're content with last generation GbE technology - the systems are identical for NAS.
(Note - I didn't check to see if the Dell 3620 has a spare slot that supports a 10 GbE card. I don't think that's relevant, however - since I'm saying "quote the price on what you need". If 10GbE is necessary, then make sure that you pick a base models that can support it and add the price of the adaptors.)
Good point - unless you still need those drives on your other Apple.
No, so please don't propose them.
If you plan on repurposing existing equipment to the workstation, then by all measures that's part of "the price for what you need".
If you have a big investment in T-Bolt peripherals that you'll move to the workstation - then for you the $3000 Apple might be more cost effective than the $1500 Dell.
If it's new storage, the four spare hard drive slots in the Dell might be more cost effective than a bunch of new external storage on a Mac Pro.