I can only speak from my experience. I originally built a square basket following the idea of the 20/20 thread posted here. I did make a change in the dimension to create a larger volume (created taller sides). I did 2 test burns wanting to confirm I could get the 14+ hour performance repeatedly reported on here...did not come close, died after 6-7 hours with plenty of fuel left. I did my first smoke, removed meat and wanted to monitor the length of burn, same results, petered out after about 7 hours with plenty of fuel left. I noted and rotated the basket in accordance with the 3 air intakes after the first disappointment (one time corner lined up with ball valve intake, one time with the center of side lined up ball valve intake) thinking this may influence, it didn't.
I now have a round basket, the first two smokes I let the barrel run after removing the meat, both times it was running 235* after 12 hours before shutting it down with fuel still remaining. I used the same type of charcoal every time (including square burns). My UDS is built to the "common" specs found here using a weber grill lid.
I am sure a much more experienced member has had great success with a square design. I did not, I now have a smoker that is AWESOME.
*Badfrog fixed my smoking world by extending his talents on providing me the round basket.