Were they named MERCY or MARCY?

Seems there is a great deal of confusion over the spelling of this female name in my Wheeler and Watson family lines in the 18^th and early 19^th centuries.

The mother of my ancestor, Desire Wheeler Watson, I thought was Mercy Williams who married Thomas Wheeler. Here is an excerpt from the American Genealogist, vole 82, 2007 pages 60-61 as shown on the NEHGS website:  

A look at the family of Mercy Wheeler, first wife of Capt. Christoper Gardner of South Kingstown, provides additional argument that Rufus Gardener of New London was likely their son.  She is presented in The Gardiners of Narragansett as “a daughter of Thomas and Mercy (Williams) Wheeler.” ……In his will of 28 November 1760, proved on 6 January 1761, Thomas Wheeler of Stonington. “in a Weak and Low Status of Body”, names his wife, Marcy, son Thomas …. Daughter Marcy Gardner ….There is no estate file for Thomas’s widow, Mercy, in Stonington.

Well, which is it ?? Seems to change back and forth between using E or A, as I have seen with others, including these two who confused me when I first found the name spelled with an A.

       Mercy Watson 1778-1817, wife of Ezra Lester

       Mercy Watson 1810-1882, wife of Col John Watson

           Her NYC death certificate & Woodlawn memorial both use A

           Seems she was named for her aunt Mrs Mercy Lester above

Today I found another one, Mercy Stanton …or was it Marcy? Looks like A wins in this case.