head to head
Neither is simply harder - they are hard in different places. Marathon demands more variance; Super Smash Bros. Melee demands more execution.

Execution · Competitive PvP
Aim duels against human crews layered over hidden-information loot gambles and extraction timing.

Execution · Competitive PvP
A platform fighter with one of the highest execution ceilings in any competitive game, layered on top of deep human reads.
Marathon leans Execution (Competitive PvP); Super Smash Bros. Melee leans Execution (Competitive PvP).
Super Smash Bros. Melee leans clearly harder on execution.
Marathon
Gunfights against other crews reward raw aim heavily
Super Smash Bros. Melee
Wavedashing, L-cancels, and tech-chasing set an extreme physical ceiling
Super Smash Bros. Melee leans clearly harder on reads.
Marathon
Predicting human crews' rotations and ambushes is constant
Super Smash Bros. Melee
Reading DI choices and opponent adaptive habits is constant and non-convergent
Marathon leans clearly harder on variance.
Marathon
Loot spawns and unseen squads create hidden information
Super Smash Bros. Melee
Stage hazard RNG and spawn positions add marginal chaos
Super Smash Bros. Melee leans a little harder on optimization, though the gap is small.
Marathon
Loadout economics and extraction routing sit underneath
Super Smash Bros. Melee
Character matchup tiers and optimal movement routing are deeply studied
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.