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 permadeath precision runner about clearing trap-filled levels in one go.

Execution · Pure Mechanics
A rally racer where mastering precise car control against tricky terrain is everything.
ALTF4 leans Execution (Pure Mechanics); Colin McRae: Dirt 2 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.
ALTF4
Permadeath precision movement through traps is the entire test
Colin McRae: Dirt 2
precise rally driving and braking required
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
ALTF4
Solo runner, no opponent
Colin McRae: Dirt 2
no adaptive opponents, mostly time trials
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
ALTF4
Traps fixed once learned
Colin McRae: Dirt 2
weather and surface conditions vary
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
ALTF4
Memorising trap layouts and clean lines through them
Colin McRae: Dirt 2
car tuning and racing lines solvable
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.