Fast TM on double legacy Machamp ?

In PvP Counter is strictly superior. Both moves have the same rate of energy gain while Counter deals more damage.

In PvE (raids/gym) Counter has 33% more DPS than Karate Chop while Karate Chop has roughly 41% more EPS than Counter. Despite this translating to using your charged move faster with Karate Chop, move sets with Counter have higher overall DPS.

By the numbers, Counter is the better attack. Having said that, I'd be pretty hard-pressed to get rid of Karate Chop purely due to legacy value.

If PvP's your game, counter is better. Though I haven't seen much about fighting being too impactive in PvP with the large number of pokemon that resist fighting being prominent, and most of the time it seems fighting types when they are in place are used for their other moves, not their fighting sets.

However, if you arent too concerned with PvP, in raids and gyms, counter and karate chop preform close, counter is slightly better most of the time, but much like draco meteor and outrage on dragonite, the differences are so small, they are negligible and could be considered equal.

Stone edge as a charge move I would keep hands down. If out clearing gyms, it does a lot agaisnt flying types like dragonite, gyarados, togekiss, and the like, and could be fun to play with in PvP, but theres other better rock moves. I'd recommend buying another charge move slot to preserve stone edge.

I'd keep both moves personally. Fighting's not much use in PvP from what ive heard, aside from countering tyranitar, but theres other better pokemon that can do that, and in raids and gyms, the two moves are close.

As for utility a bit more indeapth, gyms arent what they used to be, blissey largely remans as the sole normal tank in gyms, so machamp isnt as heavily needed as it once was, gyms are much more diverse but also much easier to beat due to the GRB nerf and stat rebalance, so having the 100% best most optimal move for every single last DPS isnt critical, KC will be more than sufficient for usage. If you're trying to build a team to solo tyranitar or absol, i'd suggest getting another machamp for counter, legacy moves are valuable purely because they are legacy, and KC can keep pace with counter in most every case.

Get the second charge move and hope for Dynamic Punch. Your Machamp is now a beast, and can decimate those Dragon/Flying types looking to feast on it.

I've got a Legacy Karate Chop / Cross chop Machamp and I'd never trade those moves. One of the few useful legacy Pokemon. Just evolve a nee Machamp?

It's legacy moves value is too damn high to change either so buy a second charge move on it and leave as is OR power up a new Machamp.

Everyone here has great answers.

I just wanted to add that Machamp isn't difficult to get at all. Tier 3 raids are fairly common and so is cloudy weather. You can get a good iv weather boosted Machamp to suit your PvP needs while keeping the legacy.

I would never TM a legacy move, no matter how useful or not it is.

I understand your dilemma is due to the fact that both moves are legacy and for making the most out of Stone Edge you would need to pair it with the Counter / Dynamic Punch (2nd) combo. However, Karate Chop is very close to Counter in PvE and you never know what will be your priorities in the future or after a rework in the game. They may introduce a new gym rework that requires much more dodging to excel, or they may nerf Counter. What you know for sure is that you'll never be able to switch it back to Karate Chop.