Price Difference
At the first glance Pro appears $80 more expensive than the base version; however, the real price difference is more significant because UDM Pro lacks AP functionality and majority of people need Wi-Fi function.
Although there is a way to make existing, non-UniFi router as AP in most cases, this is not a recommended way of setting UniFi system up because you would then have severely limited UniFi experience and functionality, and may not really have a reason to pay premium price for UniFi system compared to other consumer gears.
So you need to add a cost of purchasing at least one AP to the $80 difference.
UDM’s built in AP is a Wi-Fi 5 4×4 Wave 2.
At this time of writing, there are Wi-Fi 6 compatible access points available for the purchase. U6 Lite can be purchased for $99. So if you were to go with this AP, you are looking total of $180 more for UDM Pro + U6 Lite when compared to UDM base built in WiFi Access point.
Conclusion
Just like everything else in the world, the answer to which one is better question comes down to the context dependent. I would personally ask following questions.
Is rack mountable an option?
If answer is “no”, you need UDM base.
Do you need over 1 Gbps performance?
If answer is “yes”, you need UDM Pro.
Do you need full 1 Gbps performance while network security system (IDS/IPS and DPI) turned on?
If answer is “yes”, you need UDM Pro.
Do you need built in Access point?
If answer is “yes”, you need UDM base.
Are you looking for saving money?
UDM base is cheaper.
If you are looking for simple existing consumer router replacement, UniFi Dream Machine base is the one. If you are looking for the most scalable and powerful experience including 10 Gbps ports, 3.5 Gbps throughput even with IPS/IDS and DPI, UDM Pro is the one. You need to remember that UDM Pro lacks built in access point functionality so you need to purchase a separate access point to have UniFi WiFi experience.