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 · Pure Mechanics
Holding clean lines at speed through traffic on memorisable point-to-point roads.

Execution · Pure Mechanics
Motion-controlled tennis, bowling, golf, baseball and boxing built around simple swing gestures.
The Need for Speed leans Execution (Pure Mechanics); Wii Sports leans Execution (Pure Mechanics).
The Need for Speed leans a little harder on execution, though the gap is small.
The Need for Speed
Holding racing lines at speed through traffic is the test
Wii Sports
Motion swing timing and accuracy decide every event
Wii Sports leans a little harder on reads, though the gap is small.
The Need for Speed
AI rivals and police follow fixed behaviour
Wii Sports
Casual local opponents add light mind games
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
The Need for Speed
Traffic patterns add mild run-to-run noise
Wii Sports
Minor physics wobble and CPU behaviour randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
The Need for Speed
Track knowledge and car choice settle quickly
Wii Sports
Simple technique tricks like bowling spin are learnable
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.