Given the fact that both Machamp and Alakazam can be caught from Raids, I'd focus on trying to catch a few of them before the raid bosses change.
From the looks of it both are levels 20-21ish, so there's no savings on Stardust for evolving one or the other - if either of them were level 28+, it would be a different story. But I'd suggest trying to get those Pokemon from raids first, and only evolve to them once they are no longer available.
Even after they're no longer available, I might not evolve either - neither one of them are great as Defenders (and Gym Defense isn't important these days anyway), so that lower Atk rating is a bit of a deal breaker. Both can be hatched from 2km eggs, and both seem to be more common spawns now, so I wouldn't evolve either of them.
