The short version
Muse S Athena is the better all-rounder; Sens.ai is the right pick only when multi-modal active intervention is specifically what you want. The price gap is significant — Muse delivers most of what most users actually need at a third of the cost.
When Sens.ai is the right pick
If the EEG + photobiomodulation + HRV stack in one programme is the experience you want — and you are paying for active intervention (PBM light therapy) rather than measurement alone — Sens.ai is the right shape. Premium pricing with an ongoing subscription; high-engagement users only.
When Muse S Athena is the right pick
If you want the most mature consumer brain-training experience — meditation, focus, sleep tracking, sensor fusion (EEG + fNIRS), no mandatory subscription — Muse S Athena is the right shape. Three times cheaper than Sens.ai for most of the same job.