I have cooked in my UDS below zero as well. Keep it out of the wind, and prepare to use a little more fuel with intakes more open than usual. Search the forum, I have seen posts from a guy in Alaska cooking on his UDS on temps as cold or colder than Harbor.
I agree it's a draw. Metal thickness is close to the same with a very similar setup inside.
I've used them both well below freezing. Wind block helps. Thermal blanket (reflectex or welding blanket) will make it as though it's a normal summer day.
It was well below zero whereI live on the weekend and my UDS worked fine. I had planned to get some reflectix, but put it off 'til payday. Even without the blanket, I had no issues - I just try to keep it out of the wind.
Cooked some ribs and chicken on the 22.5" WSM yesterday. It was about 42* ( I know, not that cold) but the wind was blowing pretty hard. Hooked up the BBQ Guru DiqiQ I just got in the mail and it stayed at 250 for 5 hours with no problem. I don't have a UDS but I may build one just for the heck of it. Might be a fun project.