Shul is a Word of Power while krein is not. The word krein is only featured in the Song of the Dragonborn so it's hard to say anything about its specific meaning.

Shul on the other hand is possibly related to sul "day," giving shul a stronger connection to light (daylight), heat, and fire.

so krein is more as the actual sun and shul is more like the sun as is shines upon you?

Sure, that's a good way of putting it. So something like "the sun is shining bright" might use shul, whereas "the sun and the moon," referring to the entity of the sun itself, might use krein.

okay I will keep to that then.

thanks