Shirk is a form/type of Kufr. The one who rejects the existence of the Creator is worse in disbelief (Kufr) than a person who believes in a creator, but worships others besides him. Imam Ibn Taymiyyah (rahimahullah) said: مَن أنكر المعاد مع قوله بحدوث هذا العالم فقد كفَّرَه الله ، فمن أنكره مع قوله بقدم العالم فهو أعظم كفرا عند الله تعالى The one who denies the Hereafter but believes that this universe is created is described by Allah as a kaafir. The one who denies it and says that this universe existed from eternity is a worse kaafir in the sight of Allah, may He be exalted. Source: Majmu' al-Fatawa 17/291 (I took the translation from a fatwa of islamqa.) (I'm sure other 'Ulama` said the same regarding atheism being a bigger sin.)