head to head
Neither is simply harder - they are hard in different places. Diplomacy demands more reads; Spy vs Spy demands more variance.

Reads · Pure Mind Games
A classic strategy game of negotiation and betrayal among nations.

Reads · Pure Mind Games
A split-screen trap duel testing prediction of where your human rival hid the bombs.
Diplomacy leans Reads (Pure Mind Games); Spy vs Spy leans Reads (Pure Mind Games).
Spy vs Spy leans clearly harder on execution.
Diplomacy
no physical skill, purely negotiation and planning
Spy vs Spy
Simple movement and item fiddling under time pressure
Diplomacy leans clearly harder on reads.
Diplomacy
reading and out-negotiating human players is core
Spy vs Spy
Guessing where the rival spy laid traps is the core duel
Spy vs Spy leans clearly harder on variance.
Diplomacy
no hidden info or randomness, pure information
Spy vs Spy
Item locations are hidden information each round
Diplomacy leans clearly harder on optimization.
Diplomacy
strategic alliance and movement planning solvable
Spy vs Spy
Trap-and-counter knowledge forms a learnable layer
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.