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 · Practiced Mastery
Planning approaches with recon tools, then executing them under lethal fire.

Execution · Competitive PvP
Tactical gunfights testing aim, entry planning and prediction of human opponents online.
Tom Clancy's Ghost Recon Advanced Warfighter leans Execution (Practiced Mastery); Tom Clancy's Rainbow Six: Vegas leans Execution (Competitive PvP).
Tom Clancy's Rainbow Six: Vegas leans a little harder on execution, though the gap is small.
Tom Clancy's Ghost Recon Advanced Warfighter
Lethal firefights punish sloppy aim and exposure
Tom Clancy's Rainbow Six: Vegas
Aim and snap-to-cover gunplay punish slow hands
Tom Clancy's Rainbow Six: Vegas leans a little harder on reads, though the gap is small.
Tom Clancy's Ghost Recon Advanced Warfighter
Multiplayer squads outflank and adapt to your 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.
Tom Clancy's Ghost Recon Advanced Warfighter
Enemy placements are scripted-but-unknown on first attempt
Tom Clancy's Rainbow Six: Vegas
Terrorist hunt spawns vary modestly between runs
Tom Clancy's Ghost Recon Advanced Warfighter leans a little harder on optimization, though the gap is small.
Tom Clancy's Ghost Recon Advanced Warfighter
Drone recon, squad orders and approach planning decide missions
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.
full breakdown
Tom Clancy's Ghost Recon Advanced Warfighter →
full breakdown
Tom Clancy's Rainbow Six: Vegas →