Sennheiser - HD 660S2 vs Bose - QuietComfort Ultra 2

These two are nearly even on the Mars Score (64.2 vs 64). The decision isn't about overall quality — both clear the bar — it's about which specific trade-offs fit your use.

Pick the HD 660S2 if you care most about driver size. The biggest gaps in its favor are driver size.

Pick the QuietComfort Ultra 2 if you carry the device everywhere and weight or thickness has been the deciding factor before. Its strongest claims are weight, active noise cancellation, spatial audio, among others.

Both ship with comparable ldac, water resistance, so those specs don't separate the two — focus on the differences below.

Bottom line: there is no wrong answer here — match the spec advantages to how you actually use the product. The Mars Score parity tells you both are well-rounded; the differences are about trade-offs, not quality.