660nm and 630nm are both red wavelengths used in photobiomodulation research, but you cannot automatically say that one wavelength is better for every application.
What does nm mean?
Nanometer is the unit used to describe the wavelength of light. Both 630nm and 660nm are in the visible red spectrum.
Why different wavelengths?
Light with different wavelengths has different optical properties. Therefore, different bands are being researched for various applications and tissues.
Is 660nm proven better than 630nm?
Designating a general winner is not justified. Studies vary in purpose, dose, irradiance, treatment time, device, and population.
Why 660nm is interesting
660nm is common in PBM research and in consumer devices for skin-focused applications. The complete protocol remains decisive.
How to choose?
In addition to wavelength, also check irradiance, distance, treatment time, and the purpose for which the device is intended.
Conclusion
630nm and 660nm are not simply a winner and a loser. The relevant difference lies in the complete light protocol.
Sources: PBM-consensus and umbrella review.