Isaac son of David b. 20 Jan 1748 in Farmington, Conn. Source: White, Lorraine Cook, ed. ''The Barbour Collection of Connecticut Town Vital Records.'' Vol. 1-55. "Farmington" p.331. Baltimore, MD, USA: Genealogical Publishing Co., 1994-2002. Link at Ancestry
Southington was part of Farmington until 1799, so first Church of Southington would have been under the civil jurisdiction of Farmington in 1740's
It would not surprise me if the dates stated in both records were say 1747, and one person transcribing interpreted the date as 1746/7 and the other interpreted as 1747/8.
Short of examining the originals... your guess is as good as mine.