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
The original 2003 WWII shooter built around cinematic squad set pieces.

Execution · Practiced Mastery
A cover-shooter Rainbow Six blending tactical room-clearing with solid gunplay.
Call of Duty leans Execution (Practiced Mastery); Tom Clancy's Rainbow Six: Vegas 2 leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty
Aim and reaction carry the scripted firefights
Tom Clancy's Rainbow Six: Vegas 2
Cover-based gunfights still punish poor aim
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty
Light multiplayer prediction; campaign AI is scripted
Tom Clancy's Rainbow Six: Vegas 2
Multiplayer brings some human adaptation; AI is scripted
Tom Clancy's Rainbow Six: Vegas 2 leans a little harder on variance, though the gap is small.
Call of Duty
Some spawn and hit unpredictability
Tom Clancy's Rainbow Six: Vegas 2
Enemy placements partially unknown per encounter
Tom Clancy's Rainbow Six: Vegas 2 leans a little harder on optimization, though the gap is small.
Call of Duty
Memorising set pieces and weapon positions
Tom Clancy's Rainbow Six: Vegas 2
Room-clearing order and squad commands are solvable tactics
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.