Seasonic is one of the top psu manufactures and makes some of Corsairs best units. Corsairs budget units like the CX, VS and CS use lower quality components and are best avoided. The 620w Seasonic is a good init and will work fine. But in cpu heavy games youll need a big oc on that 6300 for it to try and keep up.
SeaSonic M12II 750W or SeaSonic SSR-650RM is a better choice than your mentioned model. As PTOWN GAMER suggested, Fx6300 is not so sufficient for powering a 390X to get maximum out of it.