When is the "main" mitzvah of hearing the Megillah?

Question

If someone has a choice of hearing the Megilla read either at night or during the day, (e.g. the person is disabled and can't make it to a shul, and no one can come to read both times) but not both, is there a halachically preferred time when s/he should hear it?

Answer 1

Tosefos on Megillah 4a (s.v. "חייב אדם") writes that the primary pirsumei nisa is in the daytime reading.

דעיקר פרסומי ניסא הוי בקריאה דיממא וקרא נמי משמע כן

The Turei Even to Megillah 7b, s.v. סעודת פורים writes that the primary enactment, from the Megillah, of reading Megillah is only during the day, and the enactment to read at night is a secondary Rabbinic extension, but is not the actual enactment of the Megillah itself.

והכי נמי לענין מקרא מגילה דכתיב הימים האלה נזכרים ונעשים נמי ימים דוקא אבל קריאת המגילה של לילה אינו מדברי קבלה

Therefore, if one were forced to choose, it would seem appropriate to prefer the daytime reading.

This holds according to Tosefos on Yoma 33a who holds that if only one of two mitzvos will be performed, then the significance of one mitzvah will override the other. However, Rashi (Mo'ed Katan end of 9a) holds that one performs a mitzvah even if it will prevent the performance of another mitzvah which is greater than it. According to Rashi, it would seem to make sense to listen to the Megillah at night, as this is the mitzvah that comes first. (Following Rashi, the Radbaz in Responsa 4:13 ruled that someone in prison, who was given one day "out" of his choice, should not wait until any particular Holiday or other time when optimal mitzvah performance could be achieved, but rather should take off for the performance of whatever mitzvah opportunity comes first.)

The Aruch HaShulchan (O.C. 687:3) indeed brings this line of reasoning regarding this exact question, and says that even though the daytime reading is primary, one would not bypass the night reading for the sake of the daytime reading because we do not bypass any mitzvah:

דאף על גב דקריאת היום הוא העיקר, מכל מקום לא יעבור על המצוה, ויקראנה בלילה שהיא קודמת, ואין מעבירין על המצות. וכן נראה לי עיקר לדינא

However, the Sefas Emes writes that the entire point of the night reading is to prepare for the day, and that in the event that there will not be a daytime reading, for whatever reason, there is no mitzvah to read at night:

ואפשר דהא דאמר ולשנותה משום דעיקר מצותה ביום היא והקריאה בלילה היא רק שיהיה ביום קריאה שניה ונפקא מינה דהיכא דביממא לא יהיה עליו חיוב גם בלילה פטור

Accordingly, it would make sense to prefer the daytime reading, for if one reads only the night reading, they end up with no mitzvah at all.