The Corsair RM650X uses a modern DC-to-DC circuit design to generate its minor (i.e. +3.3V and +5V) rails and is Intel C6/C7 (a.k.a. Intel Haswell or later) power state compatible as well as being more efficient (i.e. 80PLUS GOLD) with a 10 Year Warranty.
The Seasonic M12II 620W uses an old group regulated circuit design and isn't Intel C6/C7 (a.k.a. Intel Haswell and later) power state compatible. It only has 80PLUS BRONZE efficiency and half the warranty length (i.e. 5 Years).