I think the M18 would be easier to add because the hull of the M4 is already in game. The main gun is slightly different, but nothing too major to model I bet as it is right on par with the M4A3's 76mm.
However, the M10 really offers no advantage vs what we have now in AH. The M18 is much faster and would offer a new "category" in AH: fast and powerful, but weak in armor.