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
Stick-skill execution and constant prediction of a human opponent's attacking patterns.

Reads · Competitive PvP
The Wii entry whose slower pace and infamous random tripping pushed Smash toward defensive mind games.
FIFA Soccer 12 leans Execution (Competitive PvP); Super Smash Bros. Brawl leans Reads (Competitive PvP).
FIFA Soccer 12 leans a little harder on execution, though the gap is small.
FIFA Soccer 12
Dribbling, timed tackles and finishing demand practiced sticks
Super Smash Bros. Brawl
Slower engine lowers the ceiling but spacing still needs hands.
Super Smash Bros. Brawl leans a little harder on reads, though the gap is small.
FIFA Soccer 12
Human opponents adapt formations and attacking habits constantly
Super Smash Bros. Brawl
Defensive bait-and-punish yomi dominates its floaty neutral.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
FIFA Soccer 12
Deflections and physics bounces swing tight matches
Super Smash Bros. Brawl
Random tripping injects genuine uncontrollable RNG into movement.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
FIFA Soccer 12
Formation choices and tactical defending knowledge sit underneath
Super Smash Bros. Brawl
Matchup knowledge and ledge play reward study within a campy meta.
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.