The main difference between Muse 2 and Muse S is that the latter supports sleep tracking and the former doesn’t. Muse S also has a slightly different form factor than Muse 2, in order to make it more comfortable to wear overnight. Additionally, Muse S has a larger-capacity battery that lasts for up to 10 hours, whereas the battery in Muse 2 only lasts for about five hours. If your primary interest in Muse is meditation, you’ll be happy with either device because they both offer exactly the same meditation features. I’d even argue that the Muse 2 might be slightly easier to put on and take off (and you don’t look like a coal miner while wearing it).