I have both the 1DX and 5DIII. I had the grip for the 5DIII, but I have sold it. I use the 5DIII either when I want something smaller and lighter or when I shoot low ISO, manual focus (TS-E or Otus), often on a tripod, with RRS L-bracket. For everything else I use the 1DX. The grip was collecting dust for a number of months, so I sold it. No regrets.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1