head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is optimization, and it is a single point.

Execution · Pure Mechanics
Punishing beat-em-up execution with zero margin and memorised stages.

Execution · Practiced Mastery
Sonic alternates breakneck daytime boost stages with brawling night levels as the Werehog.
Double Dragon III: The Sacred Stones leans Execution (Pure Mechanics); Sonic Unleashed 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.
Double Dragon III: The Sacred Stones
Brutal difficulty and no continues punish every missed input
Sonic Unleashed
Daytime boost stages demand reaction-speed precision
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Double Dragon III: The Sacred Stones
Enemies attack in fixed scripted patterns
Sonic Unleashed
Scripted enemies in both forms
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Double Dragon III: The Sacred Stones
Stages never change between attempts
Sonic Unleashed
First-run stage hazards arrive fast and unseen
Sonic Unleashed leans a little harder on optimization, though the gap is small.
Double Dragon III: The Sacred Stones
Character-swap management and memorised layouts are the path through
Sonic Unleashed
S-rank routes and Werehog combat upgrades are solvable
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.