Me too. 'Brothers' was the best selling album on CD for a while - mainly because it was one of the first releases in the 'new' format. 'Dark Side' sells consistently, with increased revenue every time they bring out a new edition.
Dark side of the moon is the biggest selling album by a British band worldwide with sales of well over 25m copies. it spent over 14 years on the American Billboard chart. There is no exact figure for sales as there are huge holes is the sales records from 1973 (the original release) but it is estimated that one in every 18 Americans own a copy, I think you'll agree that is quite a lot.
"Brothers In Arms" is by far the bigger UK seller, but internationally "Dark Side..." has the edge, quite comfortably. The big difference is US sales; as noted, "Dark Side..." is one of the biggest ever sellers there.