Hirhurim just had a post about this topic, where he discussed some reasons that people should follow the aruch haShulchan. Yet in many circles, people just follow the Mishna Berura and rarely even study the aruch haShulchan for halacha. How did this develop and is it justified based on halachik principles?
I think there may be a number of factors that led to the Mishnah Berurah becoming the accepted work in many circles.
In each topic, the aruch hashulchan presents the whole sugya, which makes it better for someone learning, but not as quick if you just want the final halacha. The mishnah berurah separates out the discussion part, so its quicker to see the halacha, which may have helped it become more accepted. However, this is not a good reason to follow the MB in a case of dispute.
Also, the Aruch haShulchan is more willing to pasken in a case of dispute, while the mishnah berurah often tries to be choshesh for both sides. This may have helped the MB become more accepted in yeshivish circles. However, some may favor an approach that analyzes the sources and reaches a conclusion. The article I linked to quotes R' Henkin for some reasons that the Aruch haShulchan should be considered more authoritative.
The Lubavitcher Rebbe says that he was worthy that his sefer was accepted by all Jews since he was "a man who wanted life" (Ish Ha-chofetz Chaim).
The Lubavitcher Rebbe there suggested that people learn Shulchan Aruch Orach Chaim, the Rama, the Nosei Keilim, Alter Rebbe's Shulchan Aruch (the Shulchan Aruch Harav) and the Mishna Brura.
Just to add another point, the Mishna Berurah is the most comprehensive work in terms of analyzing all the different opinions both in the Rishonim and in the commentaries of the Shulchan Aruch and other Halachic works, and he addresses almost every issue and difficulty that arises when one learns a siman in Shulchan Aruch in depth. In this regard, his work is almost unparralleled. This is one reason why his book is learned by many. Of course in a case of a specific disagreement between the Mishna Berurah and the Aruch Hashulchan, a posek might prefer to rule like the Aruch Hashulchan, on the basis of reasoning related to each specific case.