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
The original 2003 WWII shooter built around cinematic squad set pieces.

Execution · Practiced Mastery
Stick execution and opponent prediction over a franchise-management layer.
Call of Duty leans Execution (Practiced Mastery); NHL 2K6 leans Execution (Practiced Mastery).
Call of Duty leans a little harder on execution, though the gap is small.
Call of Duty
Aim and reaction carry the scripted firefights
NHL 2K6
Shot timing, dekes and poke checks decide possessions
NHL 2K6 leans a little harder on reads, though the gap is small.
Call of Duty
Light multiplayer prediction; campaign AI is scripted
NHL 2K6
Human opponents disguise plays and bait pulls online
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty
Some spawn and hit unpredictability
NHL 2K6
Rebounds and deflections add per-game noise
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty
Memorising set pieces and weapon positions
NHL 2K6
Line management and franchise decisions reward planning
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.