head to head
Neither is simply harder - they are hard in different places. Arma 3 demands more variance; Rainbow Six Siege demands more reads.

Execution · Grand Arena
Open military simulator and PvP sandbox.

Reads · Grand Arena
A tactical breach-and-defend shooter driven by mind games over angles and gadgets.
Arma 3 leans Execution (Grand Arena); Rainbow Six Siege leans Reads (Grand Arena).
Arma 3 leans a little harder on execution, though the gap is small.
Arma 3
Bullet drop, stance, sway, weight all matter
Rainbow Six Siege
Crisp aim required but TTK is unforgiving.
Rainbow Six Siege leans clearly harder on reads.
Arma 3
PvP and milsim ops reward prediction
Rainbow Six Siege
Defenders adapt holds; attackers fake and bait.
Arma 3 leans clearly harder on variance.
Arma 3
Mission scripts and AI behavior vary
Rainbow Six Siege
Site rotations only; loadouts deterministic.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Arma 3
Loadout planning and tactics solvable
Rainbow Six Siege
Operator picks, gadget placement, and utility usage.
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.