In Gen 8, Conkeldurr is generally preferred for its higher Atk, HP, and Def. Machamp does have better speed (55 vs 45), but it’s still so slow that you’re not going to be outspeeding anything outside of Trick Room, and if you are using TR, Conk’s lower speed is better. Machamp’s main novelties are its Gigantamax form and No Guard + DPunch, the first of which is worse than its non-GMax counterpart, and the second of which is worse than Guts + Flame Orb (at which point, you should be using Guts Conkeldurr instead for its higher Atk). Conkeldurr also has Iron Fist, if you don’t want the downsides of Flame Orb, while Machamp is stuck with just Guts if you want an Atk-boosting ability.