head to head
Call of Duty: Modern Warfare 3 asks more than Tom Clancy's Rainbow Six: Vegas, and the gap is execution.

Execution · Pure Mechanics
A fast-paced military shooter where twitch aim against other players is the core test.

Execution · Competitive PvP
Tactical gunfights testing aim, entry planning and prediction of human opponents online.
Call of Duty: Modern Warfare 3 leans Execution (Pure Mechanics); Tom Clancy's Rainbow Six: Vegas leans Execution (Competitive PvP).
Call of Duty: Modern Warfare 3 leans clearly harder on execution.
Call of Duty: Modern Warfare 3
aim and reflexes decide most engagements
Tom Clancy's Rainbow Six: Vegas
Aim and snap-to-cover gunplay punish slow hands
Call of Duty: Modern Warfare 3 leans a little harder on reads, though the gap is small.
Call of Duty: Modern Warfare 3
predicting human opponent positioning and habits
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.
Call of Duty: Modern Warfare 3
spawn placement and loadout drops vary
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.
Call of Duty: Modern Warfare 3
loadout tuning and map knowledge help
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.