head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Execution · Practiced Mastery
Precise shot aiming plus planning the run-out several balls ahead.

Execution · Practiced Mastery
A cover-shooter Rainbow Six blending tactical room-clearing with solid gunplay.
8 Ball Pool 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.
8 Ball Pool
A perfect aim-line cheat wins nearly every rack outright
Tom Clancy's Rainbow Six: Vegas 2
Cover-based gunfights still punish poor aim
Tom Clancy's Rainbow Six: Vegas 2 leans a little harder on reads, though the gap is small.
8 Ball Pool
Opponents alternate turns; little adaptive interaction to predict
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.
8 Ball Pool
Break spreads add some randomness to each rack
Tom Clancy's Rainbow Six: Vegas 2
Enemy placements partially unknown per encounter
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
8 Ball Pool
Position play and run-out planning separate good players
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.