shulchanhashalem:
The way I see it, there are several reasons to prefer the Aruch Hashulchan:
1) He was a practicing Rabbi, unlike the Chofetz Chaim.
2) He had access to the Mishnah Berurah (I don’t think it’s relevant how often he actually quotes him.)
3) His work covers all of halacha (except for a few bits which are missing) whereas the Mishnah Berurah is only on Orach Chaim.
4) The Mishnah Berurah often just quotes opposing views of the acharonim without deciding between them.
5) The Mishnah Berurah generally just explains the Shulchan Aruch/Rema (and quotes the opinions of later acharonim). Whereas the Aruch Hashulchan to a certain extent takes you through the sugya and explains his reasoning and brings rayas to his positions. This point kind of relates to: http://www.theyeshivaworld.com/coffeeroom/topic/paskening-from-the-shulchan-aruch-without-knowing-the-gemara-and-rishonim
(One of these days I’ll get around to updating my list of sources there.)
6) As I quoted earlier in this thread, the Mishnah Berura’s methodology was generally to count up the acharonim (perhaps granting extra weight to certain of them) which often resulted in conclusions that were against the prevailing practice of the times.