head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Pure Mechanics
Dodging scripted bullet patterns with stage memorisation doing the planning.

Execution · Pure Mechanics
Retro Studios' notoriously tough revival of Donkey Kong Country with relentless set-piece levels.
Air Buster leans Execution (Pure Mechanics); Donkey Kong Country Returns leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Air Buster
Bullet weaving and tight corridors demand precise control
Donkey Kong Country Returns
Demanding rocket-barrel and mine-cart sections, strict timing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Air Buster
Enemy waves are fully scripted patterns
Donkey Kong Country Returns
Scripted hazards, no adaptation
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Air Buster
Spawns are deterministic once a credit is studied
Donkey Kong Country Returns
Hidden puzzle pieces, fixed once found
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Air Buster
Memorising stage layouts and power-up routes drives clears
Donkey Kong Country Returns
KONG letters and time attack routes 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.