Connectivity
Technically, a router/gateway unit role is to connect local area network to the internet. Everything else may be considered extra-function. However, if you are like myself and coming from consumer router background, built in access point (AP) and built in switch functionality feel a de facto standard.
Gigabit WAN
WAN port is what you connect to the modem unit like cable modem. So any internet traffic will pass through the port.
Both have a single 1 gigabit RJ45, regular ethernet wide area network connection port. However, UDM Pro has additional option of 10 Gbps SFP+ WAN port.
What this means is if you have over 1 Gbps internet service, you can utilize this port and get beyond gigabit speed. This may be a critical decision factor for some and the other totally irrelevant. The question here is simple, do you have or expect over 1 Gbps internet service within the life time of this unit?
In fact, you can technically benefit from 10Gbps WAN port even with a one full-gigabit service. For example, while I still had 1 Gigabit download service through Xfinity, I had changed my modem to N-BaseT capable Arris Surfboard S33 modem. This new modem allowed me to connect the modem and UDM Pro via its SFP+ port to reach up to 2.5 Gbps speed.
Since internet service provider serves more throughput than what they are promising, using the beyond gigabit link I got 1.2 Gbps download speed on my Gigabit service. This was a jump from original 940 Mbps speed cap on the same service.
Xfinity has now bumped the service to 1.2 Gbps rating officially, so I now actually have closer to 1.3 Gbps.
WiFi Connection
As discussed already, UDM base has built in access point. Specifically, the current version has Wi-Fi 5 Wave 2 access point (AP).
However, UDM Pro lacks access point functionality. Since I assume most of us need WiFi, this means you will need to buy additional access point unit.
Gigabit LAN Switch
Regarding to the wired connectivity, both UDM base and UDM Pro have gigabit ethernet port switch built in with Pro having twice more ports than the base version.
However, UDM Pro switch has limitation of 1 Gbps backplane throughput (ref). What this means is that it is not suited for performance conscious use as a true switch ports.
Thus, I personally consider twice gigabit ethernet ports of Pro as an insignificant.
10Gbp LAN
The real connectivity difference between UDM base and Pro again comes from SFP+ ports on the UDM Pro. UDM base does not have any 10 Gbps connectivity. In addition to 10 Gbps connection on WAN side, UDM Pro has LAN side 10Gbps connection.