Physical form factor
The form factor may be a simple decision or limitation factor.
Dream Machine Base
Dream Machine base has an non-obstrusive look. I think this unit can be placed anywhere in home without worrying spouse acceptance factor.
I certainly was not able to say the same with my previous router unit from the Asus.
Dream Machine Pro
On the other hand, Dream Machine Pro has a rack mount form factor. Those who have dedicated network section in your home with a rack, the rack mountable form factor is considered as a pro; however, for everyone else, this can be a limiting factor.
Its size and form factor will not sit on kitchen counter, desk nor living room corner.
Additionally, there is also a potential noise factor.
UDM Pro is not necessary extra-loud device for its form factor; however, it has built in fan and expect it to kick in at times. Since this is a network router, most of you will have this powered on 24/7, so any noise can be a source of annoyance if it is placed in the sound sensitive area like bedroom.
The take home message here is if you do not have a space for rack-mountable device, UDM Pro is not going to be your option.
if you do not have a space for rack-mountable device, UDM Pro is not going to be your option.