The 1852 LDS edition accidentally changed the singular advantage to the plural advantages. The editors for the 1920 LDS edition restored the correct singular to the LDS text.
The singular seems more appropriate in English. The Book of Mormon text has nine other cases of the singular advantage, but only one of advantages—and that one is preceded by the plural quantifier many: “the Gaddianton robbers did gain many advantages over them” (3 Nephi 2:18). Since the singular is clearly preferred, there is no reason to reject it here in Helaman 1:25. The change to the plural in the 1852 LDS edition was very likely a typo.
Summary: Maintain the singular advantage in Helaman 1:25, especially since this is the reading of both manuscripts and all the early editions.