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 · Pure Mechanics
Tests twitch aim and physics-prop improvisation against human opponents.

Execution · Competitive PvP
Tactical gunfights testing aim, entry planning and prediction of human opponents online.
Half-Life 2: Deathmatch leans Execution (Pure Mechanics); Tom Clancy's Rainbow Six: Vegas leans Execution (Competitive PvP).
Half-Life 2: Deathmatch leans a little harder on execution, though the gap is small.
Half-Life 2: Deathmatch
Aim, movement and gravity-gun catches decide every duel
Tom Clancy's Rainbow Six: Vegas
Aim and snap-to-cover gunplay punish slow hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life 2: Deathmatch
Predicting human spawns, dodges and prop throws matters constantly
Tom Clancy's Rainbow Six: Vegas
Multiplayer angles and pushes demand reading human opponents
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Half-Life 2: Deathmatch
Physics props add minor situational chaos
Tom Clancy's Rainbow Six: Vegas
Terrorist hunt spawns vary modestly between runs
Tom Clancy's Rainbow Six: Vegas leans a little harder on optimization, though the gap is small.
Half-Life 2: Deathmatch
Weapon spawns and map control are learnable layers
Tom Clancy's Rainbow Six: Vegas
Room-entry planning and map knowledge are studyable
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.