The changes with the newest version include quieter buttons and higher sensitivity levels, which is helpful for 4k displays with much higher pixel densities requiring increased sensitivity levels to produce cursor movements at a comfortable speed. It's also the first mouse in the lineup using Logitech's newly developed BOLT receiver, designed to provide improved wireless security and performance in congested signal environments.
If you're interested in something more ergonomic, the Logitech Lift is a vertical mouse that you hold in a 'handshake position.' This design is very comfortable and may be ideal if you've had problems with more conventionally-shaped mice. It doesn't have all the productivity features as the MX Master mice, but it's less expensive, is better suited for smaller-sized hands, and there's a left-handed version available.