head to head
Halo 2 asks more than Half-Life 2, and the gap is reads.

Execution · Pure Mechanics
Landmark FPS built on physics puzzles and tight gunplay.

Execution · Pure Mechanics
The sequel that built console online multiplayer around ranked Battle Rifle duels.
Half-Life 2 leans Execution (Pure Mechanics); Halo 2 leans Execution (Pure Mechanics).
Halo 2 leans clearly harder on execution.
Half-Life 2
Gravity gun and shooting demand precision.
Halo 2
BR precision and movement tricks online
Halo 2 leans clearly harder on reads.
Half-Life 2
Combine AI scripted, non-adaptive.
Halo 2
Ranked duels hinge on outplaying humans
Halo 2 leans a little harder on variance, though the gap is small.
Half-Life 2
Almost no randomness.
Halo 2
Matchmaking opponents and spawns vary
Halo 2 leans clearly harder on optimization.
Half-Life 2
Linear progression with no builds.
Halo 2
Map control, timers, and weapon spawns
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.