Hacham Bension Abba Shaul (Israel, 1923-1998), in Or Le’sion (vol. 4, p. 319; listen to audio recording for precise citation), addresses the question that arises in a case where a person can read the Megila only once over the course of Purim – either at night or during the day. Of course, Halacha requires reading the Megila both on the night of Purim and during the day, as explicitly codified in the Shulhan Aruch (Orah Haim 687:1). It could happen, however, that a person must choose one or the other, such as if he must travel for some urgent matter, or undergo an urgent medical procedure, and he can schedule the trip or the procedure for either Purim night or Purim day. Which option should he choose?
Hacham Bension suggests that this question might hinge on the debate between the Shulhan Aruch and the Rama (Rav Moshe Isserles of Cracow, 1525-1572) regarding the recitation of "She’hehiyanu" over the Megila reading. The Shulhan Aruch rules that "She’hehiyanu" is recited only at the nighttime reading, which is the first time the Misva is fulfilled since the previous Purim. Just as we recite "She’hehiyanu" over the Sukka on the first night of Sukkot and do not repeat it the following day, similarly, according to the Shulhan Aruch, we recite "She’hehiyanu" over the Megila reading at night, and do not repeat the Beracha on Purim morning. The Rama, however, disagrees. Based on the ruling of Tosafot in Masechet Megila, the Rama maintains that the primary "Pirsum Ha’nes" (publicizing of the miracle) occurs on Purim day, and therefore "She’hehiyanu" is recited again over the daytime reading. Our custom follows the ruling of Maran in Shulhan Aruch, whereas Ashkenazim follow the Rama’s position and repeat the Beracha of "She’hehiyanu" on Purim morning.
Seemingly, Hacham Bension notes, this debate would affect the question of which of the two readings should be given precedence when one reading must be chosen over the other. According to the Shulhan Aruch, both readings are on equal Halachic footing, and therefore one should choose the nighttime reading, the obligation which sets in first. According to the Rama, however, the daytime reading is the more important of the two readings, and thus if one must choose one over the other, he should give precedence to the daytime reading. Hacham Bension then notes that this question was already addressed by the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908), who concludes that even for Ashkenazim, who follow the Rama’s ruling, one should seize the first opportunity that presents itself – the nighttime reading – rather than wait until the morning. Even according to the Rama, who viewed the daytime reading as the more important of the two readings, it is preferable to read the Megila at night in order to seize the Misva, rather than choose the later of the two readings.
Summary: If a person must choose on Purim between the nighttime Megila reading and the daytime Megila reading – such as if he has an urgent trip or medical procedure that needs to be scheduled for either Purim night or Purim day – he should read the Megila at night.