head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare 2 demands more variance; Nidhogg demands more reads.

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

Reads · Competitive PvP
Minimalist fencing duel that is almost entirely about out-guessing one human.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Nidhogg leans Reads (Competitive PvP).
Call of Duty: Modern Warfare 2 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
Nidhogg
Sword-height changes and dive-kicks need quick hands
Nidhogg leans clearly harder on reads.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Nidhogg
Pure human duels of feints and conditioning
Call of Duty: Modern Warfare 2 leans clearly harder on variance.
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Nidhogg
Symmetric deterministic arenas
Call of Duty: Modern Warfare 2 leans clearly harder on optimization.
Call of Duty: Modern Warfare 2
Killstreak economy, loadout and map knowledge
Nidhogg
Small toolkit, little to solve beyond fundamentals
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.