The main difference is size (9.4” vs 7.7” rotor disc). The MCP S also has an optional brushless upgrade motor, which makes for a nice upgrade. The rotor head and swash are also totally different.
Since they share many of the same electronics, the nano has more power for its size, and quicker response. However, due to its small size, it’s pretty much exclusively an indoor heli. The MCP won’t have the same punch, but will be a bit more stable, and can handle flying outside in low winds.