What is the preferred position for the reading of the morning and nighttime Shema – standing, or seated?
As a general rule, it is preferable to sit during the recitation of Shema, as sitting allows one to more easily concentrate on the meaning of the words. However, the Mishna Berura (commentary to the Shulhan Aruch by Rav Yisrael Meir Kagan of Radin, 1839-1933) writes that if one comes to the synagogue late for Arbit, while the congregation is already reciting Shema, he should not sit down for the Shema recitation, and should instead recite it while standing. The view of Bet Shammai, which is not accepted as Halacha, is that one must stand for the morning Shema, and either sit or recline for the nighttime Shema. If a person specifically sits down for the recitation of Shema during Arbit, he appears as though he is going out of his way to follow the view of Bet Shammai. In order to avoid this impression, one who is already standing should remain standing for Shema, rather than specifically sit down. The Mishna Berura cites this ruling from the Bayit Hadash (Rav Yoel Sirkis, 1561-1640) and Peri Megadim (Rav Yosef Teomim, 1727-1792).
However, this applies only during Arbit. For the morning Shema, one may sit down even if he had been standing before the Shema recitation. Since Bet Shammai actually requires standing for the morning Shema, one clearly does not appear as following this view by specifically sitting for the morning Shema.
Summary: Generally, it is preferable to sit, rather than stand, for the recitation of Shema. However, at Arbit, if one had been standing before Shema, he should not specifically sit for the recitation.