Q. It says in Mishna Berura that you have to wear Shabbos clothing on Purim, But it also says that you should wear an attire (partzufin) Which is more correct?
A. Different reasons have been mentioned for the minhag of donning disguises in Purim, such as giving the opportunity to the poor to collect tzedaka without being embarrassed (Minhagei Kol Aryeh)
The miracle of Purim was disguised in ordinary occurrences and events, such as Vashti being executed and Esther taking her place. The Talmud (Megila 12a) writes that just as the Jews at the time pretended only to be serving other gods, Hashem also pretended that He was going to destroy the Jewish nation, and in the end it was only a semblance (Bnai Isoschor). It would seem that we are being encouraged to wear them. However, Rema (Shulchan Aruch O.H. 696: 8) and Mahari Mintz (17) mention only that it is permitted, but do not encourage it necessarily.
While in regards to wearing Shabbos clothing on Purim, Mishna Berura (695: 3) writes in the name of the (Poskim) Achronim that it is proper to wear them at the night megila reading also.
The minhag of the Maharil, Avodas Yisroel, Ben Ish Chai (Tetzave) and others was similar (See Yemei Hapurim 5: 1).
Eliahu Rabba (691: 15) mentions that one should change to clothing of import as a remembrance to; “Mordechai left the king's presence with royal raiment, (Esther 8: 15).”
Horav Yaakov Hirschman Shlit’a suggested that one could dress up with Shabbos Chasidishe clothing, since for one who does not usually wear them, they would constitute a disguise.
Horav Shlomo Miller’s Shlit’a opinion is that wearing Shabbos clothing supersedes the minhag some have of wearing a costume, at least most of the time.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a