head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Pure Mechanics
A rally racer where mastering precise car control against tricky terrain is everything.

Execution · Pure Mechanics
A high-intensity wave shooter testing sustained accuracy and stamina against unrelenting scripted enemy hordes.
Colin McRae: Dirt 2 leans Execution (Pure Mechanics); Serious Sam 3: BFE leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Colin McRae: Dirt 2
precise rally driving and braking required
Serious Sam 3: BFE
Iron-sights mechanics add precision demand on top of classic crowd control
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Colin McRae: Dirt 2
no adaptive opponents, mostly time trials
Serious Sam 3: BFE
All enemies follow scripted wave patterns throughout the campaign
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Colin McRae: Dirt 2
weather and surface conditions vary
Serious Sam 3: BFE
Encounter layout is consistent and fully learnable after one pass
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Colin McRae: Dirt 2
car tuning and racing lines solvable
Serious Sam 3: BFE
Weapon economy and grenade usage offer a thin planning layer
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.