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 · Practiced Mastery
Coordinating two-man cover tactics and aggro swaps through scripted firefights.

Execution · Practiced Mastery
Plannable stealth routing punctuated by precise ballistic marksmanship.
Army of Two: The 40th Day leans Execution (Practiced Mastery); Sniper Elite III leans Execution (Practiced Mastery).
Sniper Elite III leans a little harder on execution, though the gap is small.
Army of Two: The 40th Day
Cover shooting needs competent aim but generous systems forgive
Sniper Elite III
Ballistic long shots demand steady, compensated aim
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Army of Two: The 40th Day
Enemy AI is scripted, co-op partner cooperates rather than opposes
Sniper Elite III
Enemy AI follows mappable alert and search routines
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Army of Two: The 40th Day
Encounters are scripted with minor spawn unknowns first time
Sniper Elite III
Patrol details and wind vary slightly per attempt
Sniper Elite III leans a little harder on optimization, though the gap is small.
Army of Two: The 40th Day
Aggro management and weapon upgrades reward planned coordination
Sniper Elite III
Open maps reward planned routes, sound masking and relocation
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.