from Wikipedia concerning AMD Instinct M25 : Radeon Instinct - Wikipedia
Here is Radeon Instinct mi25 datasheet from AMD (PDF Format): https://instinct.radeon.com/_downloads/radeon-instinct-mi25-datasheet-15.6.17.pdf
Well as a key point the Radeon (RX), Radeon Pro (WX) and Radeon Instinct (MI) Variants are on-paper identical.
This means that the RX VEGA 64 (£500), WX 9100 (£1,800) and MI25 (£3,200) are functionality speaking the same Hardware.
Where they differ however is primarily in their Binning.
In essence the MI25 is guaranteed to operate at 1500MHz underload., and will do so at ~65c while drawing no more than 230w.
Of course it's expressly designed for a Server Blade, so has no Display Ports; it also features 32GB (HBM2) VRAM.
The WX9100 has 16GB (HBM2) VRAM, and is guaranteed to Avg. 1500MHz underload., and again will do so at ~65c while drawing no more than 265w.
It does have Display Ports, 6 in Total. It also operated with the Professional Drivers, which go through additional QA Process to ensure Stability... Retail Drivers, typically are more frequently pushed out with less QA in order to keep a fairly rapid development and update Cycle but this also results in some major issues from time-to-time; something that simply would be unacceptable for Professional Environments.
RX VEGA 64 obviously has the least VRAM (8GB HBM2),. plus it's only guaranteed to hit 1500MHz while remaining under 95c and use less than 375w.
On top of this you have 2 Display Ports and 2 HDMI Ports. It's also unable to use the Professional Drivers, although arguably you might be able to use a Softmod in order to switch between the two.
Now as a keynote,. the Professional and Retail Drivers DO have very noticeably performance differences between Gaming and Workstation Workloads alike.
Typically you'll see between 15-20% Performance Difference trying to do these tasks on the "Wrong" Driver.
As such in essence you're paying for the Larger Memory, Higher Quality Control and Professional Driver Support... well that and the Warranty Guarantee., because strictly speaking both the WX and MI are "AMD" Products where-as the RX are AIB Partner Products; thus are covered by 3rd Party Warranties as opposed to AMD.
It also should be noted that you can't actually buy Radeon Instinct at Retail., instead they're Sold to Official Partners, who provide them as part of Pre-Built Server Blades. So, it's somewhat a moot point to even compare them; given either you need a Render / Compute / Build Farm., or you don't.
The only real confusion should be between WX and RX., which is legitimate as functionally they're identical and AMD even provide switchable Modes (Compute / Gaming) which switches between Pro / Retail Modes; for all intended purposes.
I've also noted (in the past) that if you play to run Multiple Cards., the "Blower" Style is better given how close they are to each other, where-as the "Axial" are better for Single Card / High-Performance Systems.