Both of those are secondary sources from long ago, so neither one of them is exactly the gold standard of information. If they disagree, I'd look for the records. Unfortunately, there are gaps in the records...
Salisbury Vital Records at https://www.americanancestors.org/DB190/r/141818208 has Nath[anie]ll Abbott and Mehetabel Collins marrying on Feb. 15, 1750. I think that Lemuel Abijah Abbott (compiler of the Abbott Genealogy) might have been speculating that this record was erroneous -- that it was Nehemiah Abbott who married Mehetabel on that date, but his name was misrecorded. It's possible that he couldn't find a Nathaniel of the right age in that area, but he found an appropriate Nehemiah. I note that Andover Vital Records at https://www.americanancestors.org/DB190/i/7646/31/22241849 show that Nathaniel and Penelope Abbott had a son Nathaniel in Andover, Mass., on 10 March 1726/7 -- the right age to have married in 1750, but the same couple had another son Nathaniel in Andover in 1737 -- see https://www.americanancestors.org/DB190/i/7646/32/22241987 , which suggests that the earlier son Nathaniel had died. (ADDED: But Concord, New Hampshire, Vital Records cite 10 March 1726, in Andover, for the birth of Nathaniel Abbott, who died in Concord in 1806.)
Lemuel Abijah Abbott described a daughter of Nehemiah and Mehitabel named Abigail, who had John Abbott appointed as her guardian after her father died. At https://www.americanancestors.org/DB515/i/13744/105-co2/30023659 I found an Essex County probate record for the guardianship case (in 1775) of Abigail, daughter of Nehemiah Abbott, but the file is indexed under the name Nathaniel Abbott (and it doesn't name the child's mother, so it's no help there).
More research is needed.
