head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto: San Andreas demands more optimization; MDK demands more execution.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.

Execution · Practiced Mastery
Precision sniping and arena navigation through fixed, gimmick-laden levels.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); MDK leans Execution (Practiced Mastery).
MDK leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
MDK
Sniper-mode precision and arena dodging demand steady hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
MDK
Alien enemies behave on fixed scripts
Grand Theft Auto: San Andreas leans a little harder on variance, though the gap is small.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
MDK
Levels are authored with nothing random
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
MDK
Learning arena gimmicks and item use solves each level
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.