head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Competitive PvP
A platform fighter with one of the highest execution ceilings in any competitive game, layered on top of deep human reads.

Execution · Grand Arena
Frame-perfect execution fused with constant prediction of an adapting human opponent.
Super Smash Bros. Melee leans Execution (Competitive PvP); Ultra Street Fighter IV leans Execution (Grand Arena).
Super Smash Bros. Melee leans a little harder on execution, though the gap is small.
Super Smash Bros. Melee
Wavedashing, L-cancels, and tech-chasing set an extreme physical ceiling
Ultra Street Fighter IV
One-frame links and FADC timing set a brutal hands ceiling
Super Smash Bros. Melee leans a little harder on reads, though the gap is small.
Super Smash Bros. Melee
Reading DI choices and opponent adaptive habits is constant and non-convergent
Ultra Street Fighter IV
Footsies and conditioning the human opponent is the core duel
Super Smash Bros. Melee leans a little harder on variance, though the gap is small.
Super Smash Bros. Melee
Stage hazard RNG and spawn positions add marginal chaos
Ultra Street Fighter IV
Almost nothing hidden once both characters are known
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Super Smash Bros. Melee
Character matchup tiers and optimal movement routing are deeply studied
Ultra Street Fighter IV
Frame data, matchups and option-selects form deep studied theory
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.