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

Execution · Practiced Mastery
Claustrophobic post-apocalyptic shooter where bullets double as currency.

Optimization · Practiced Mastery
A tense survival horror where one bite kills your survivor for good.
Metro 2033 leans Execution (Practiced Mastery); ZombiU leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Metro 2033
scarce-ammo gunfights punish sloppy aim
ZombiU
Deliberate cricket-bat timing and panic inventory management
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Metro 2033
mutants and bandits follow scripts
ZombiU
Zombies shamble on fixed logic
ZombiU leans a little harder on variance, though the gap is small.
Metro 2033
hidden moral points and supplies first run
ZombiU
Permadeath runs face unknown spawn placements
ZombiU leans a little harder on optimization, though the gap is small.
Metro 2033
ammo-as-currency economy and stealth routing
ZombiU
Route knowledge, supply caching, safehouse planning
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.