head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare 2 demands more variance; Street Fighter III: 3rd Strike demands more reads.

Execution · Competitive PvP
Blockbuster 2009 sequel famous for killstreak chains and frantic multiplayer.

Reads · Competitive PvP
A parry system that elevates reads to the central skill axis, with execution demands that remain punishing decades later.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Street Fighter III: 3rd Strike leans Reads (Competitive PvP).
Street Fighter III: 3rd Strike leans a little harder on execution, though the gap is small.
Call of Duty: Modern Warfare 2
Fast time-to-kill rewards raw aim and reflex
Street Fighter III: 3rd Strike
SA combo links and precise parry timing set an extreme execution ceiling
Street Fighter III: 3rd Strike leans clearly harder on reads.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Street Fighter III: 3rd Strike
Parry is a pure real-time read against an adapting human - the defining mechanic
Call of Duty: Modern Warfare 2 leans clearly harder on variance.
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Street Fighter III: 3rd Strike
No RNG; all information is transparent
Street Fighter III: 3rd Strike leans a little harder on optimization, though the gap is small.
Call of Duty: Modern Warfare 2
Killstreak economy, loadout and map knowledge
Street Fighter III: 3rd Strike
Super Art selection and character punish routing are well-studied
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.