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They play differently. Conkeldurr is walled 100% by Gliscor but also functions as an excellent Excadrill check with his powerful Mach Punch, while Machamp is harder to wall and had the Dynamicpunch thing going on but overall has less power and bulk than Conkeldurr.
COnkeldurr's a lot more common competitively, but I think that's more because people are underestimating Machamp and saying he's outclassed by him. He can't compete as a Bulk Up user, sure, but at nearly everything else Machamp performs very well since he doesn't have a common 100% switch-in (thanks to elemental punches).