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

Execution · Practiced Mastery
Precision sniping and arena navigation through fixed, gimmick-laden levels.

Execution · Practiced Mastery
Mid-demand 3D platforming execution across fixed gravity puzzles.
MDK leans Execution (Practiced Mastery); Super Mario Galaxy leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
MDK
Sniper-mode precision and arena dodging demand steady hands
Super Mario Galaxy
Gravity-bending jumps need timing, though forgiving by Mario standards
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
MDK
Alien enemies behave on fixed scripts
Super Mario Galaxy
Bosses repeat fixed patterns - nothing adapts
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
MDK
Levels are authored with nothing random
Super Mario Galaxy
Galaxies are scripted; surprises evaporate after one visit
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
MDK
Learning arena gimmicks and item use solves each level
Super Mario Galaxy
Star routes and comet challenges reward learned solutions
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.