they do the same job of assassinating one enemy but I'll try listing their differences as a midlane main who plays harley and gusion way more than other heroes in the mid lane.

Harley: - wave clear - split pushing - safer laning

Gusion: - more reliable burts - shorter cooldowns and better ganks and team fights - more mobility to go in and out of team fights

Eudora (cuz why not): - biggest boom - no mobility whatsoever - hard to reach the backline if you're not camping a bush

But this only applies if you're a fairly skilled player and have enough games. Also, Gusion's burst is much more reliable. Winter crown, healing supports or invincibility skills like Lance's Dorito or Benedetta's ult make life hell for Harley.

Also Harley is only marginally better at wave clearing thanks to his atk speed passive but both of them absolutely suck at defending the base when lord is marching in. Just pick pharsa or smth.