The conflicting grandfathers should not be hard to sort out. David Simpson (http://www.wikitree.com/wiki/Simpson-8389) is pretty clearly an error. John Simpson is the correct one.
Below from the Ulysses S. Grant Presidential Library (http://www.usgrantlibrary.org/about/genealogy.asp#jesseroot):
"Jesse Root Grant, b. near Greensburgh, Pa., Jan. 23, 1794; d. at Covington, Ky., June 29, 1873; m. at Point Pleasant (or Bethel), Ohio, June 24, 1821, to Hannah Simpson [b. in Montgomery Co., Pa., 20 miles from Philadelphia, Nov. 23, 1798 (or 1799); d. at Jersey City, N. J., May 11, 1883; daughter of John Simpson and Rebecca Weir."
My added emphasis.