Which Machop to Evolve?

The 91% one - the difference in level isn't that much to compensate the IV gap imo.

Not much in Gen1. Lapras/cloister/dewgong, lick snorlax, golem (esp w/ rock throw). He may have a few more favorable match ups, but you likely have better options for them. For some reason, most fighting moves are ridiculously weak compared with other types. Fighting types will be a bit more useful against the dark and steel types set to be released with gen 2.

I found Machamp barely mediocre as an attacker. Having said that I powered one up with KC/Submission to over 2500cp and he's sat in a gym for almost two weeks... His cp secures a decent spot in some local gyms and his best counters aren't well known.

Worth up a KC-SE Machamp 93.3%?.

level 20-24 is around 23 candies (plus stardust).

20 -> 21.5 = ~1750 cp
24 -> 25.5 = ~1985 cp

at the same level, the 91IV one will have about 100 more cp

the 60% is the answer now, but the 91 is more long game.