A few thoughts on this very geshmak thread!
May I say that a thread like this that “hondles” an issue that is relatively recent, and one that people can bring direct sources is the ideal thread – as opposed to other topics.
1- Anyone who has learned both knows that both are amazing. The Mishnah Berurah is far easier to read, especially if you are learning Tur/Beis Yosef, then the Shulchan Aruch as the MB is right off the Shulchan Aruch. The Aruch Hashulchan is nice because of its background to the sugya, for example it often gives not just explanations of halacha but of the pshat in the Tur, Bais Yosef, Rambam etc. And it quotes common practice, which is l’halacha significant and interesting. Assuming you want to learn the basic of halacha, the M’B is for sure the first stop. Again, one cannot posken everything from any sefer, but to get a background and the different opinions in a brief way than the M’B is more well suited (for example for bar mitzvah age children).
2- I am not disagreeing with importance of minhagim. A chassidish Rav I know (and love) often would lament that the Yeshivish world stopped certain minhagim (or at least does not allow for different minhagim) within its current system [and at time makes those minhagim to be shvach or bdeived]. Similarly, I think this what Rav Schacter is in a way saying that M’B ignores comon practice when formulating his p’sak; rather basing it on a halachic analysis solely (or mostly.
However let me say (I am not disagreeing, just pointing out something) that if one studies halacha like Tur/Bais Yosef, we see countless and countless times that there are opinions (often the accepted practiced of that Rishon/Achron and his community) that are not followed and highly questioned by practically “everyone on the page” and “all the poskim” (of course this phrase is rarely used literally). This is often with an opinion based on a unique/self-created idea or to back a practice by backing into a sevara (ie now that I know what the practice is, let me relearn the gemara). This happens for example with the Rema multiples times. NOW there are always those that find backing to the Rema for example, and I am not of course disagreeing (CHAS V’SHALOM)- but it’s just a fact that we see classical commentaries more than not disagree with unique p’sak that at least outwardly go against the gemara and its accepted interpretations. For example, the Taz says something based on his own intuition everyone jumps it. There is a sefer from the classical Minhagei Ashkenaz, and countless rishonim line up to bring proof that it is wrong (there are of course those like the Bach or Taz who will often counter). The main point of this paragraph is as follows: In hundred years, Mashgiach should be here, but assuming Eliyahu Hanavi is not paskening yet (G-d forbid) – the sefer that is more “halachic based” with fewer chiddushim will be the more accepted sefer in general. The Shach is a great example vs the Taz. The Mechaber vs. Rema (again not always or even most for Ashenazim but the amount of times most Ashkenazim do not follow the Rema I believe (please note this is not scientific) is much more than Sefardim not following the mechaber).
3- Finally, the Posek I am close to had a yesodisdik line when he gave a kitzur shulchan aruch shiur. He said over a kitzur, noted that mishnah berurah disagreed. And commented that l’masseh (not to say he was going like one opinion or the other) when you get to shmayim and you say you did so and so because the Kitzur Shulchan Aruch said so – you don’t have to worry. That is if one is basing themselves on an accepted sefer (MB ArHs Kitzur etc – it takes a long time to learn all the seferim cover to cover) that is not what we need to be worrying about in our din v’cheshbon. There are many more significant things that (at least myself) regarding middos etc. that we do not have what to be somech.
Frliechen Chanukah.