The Mark II is largely seen as addressing the flaws of the Mark I, namely hot restart issues (carb got heat soaked and some saws were hard to restart), autotune issues (system getting confused by idling too long or making many small cuts, bogging off idle), other heat issues, air filter doesn't seal great, etc.
They went through many iterations of carburetors/coils, with different fuel line setups, adding a hole in the top cover to help it cool, etc.
Your saw, being a 2018, should have the latest/last updates.
I tried a Mark II thinking I'd replace my 2017 Mark I, and it was better in every way (no decomp to plug, nicer recoil, great air filter, autotune is much better, balance is great) except that it's larger physically and also heavier. Halfway towards a 562xp sort of feel, not the super-slim Mark I feel.
I'm keeping my Mark I; it's small, zippy, and despite the autotune bumbling about now and again it's a better fit for me than a Mark II.