Their difference is the same like the difference of the Sun and the moon.
sadhana and by self-enquiry or deep meditation and samadhi his buddhi merges in consciousness and becomes identical with consciousness (Brahman)
“The pure mind is itself Brahman; it therefore follows that Brahman is not other than the mind of the sage.”
― Ramana Maharshi
Thus the buddhi of a Buddha is illuminated; viz. it is free from egoism, desire, fear, anger, like-dislike, atraction-repulsion, attachment and suffering; it is pure sattva and identical with consciousness, experiencing peace and ananda effortlessly.
While the buddhi of a deluded is full of rajas and tamas, egoism, desires fear, anger, likes, dislikes, attchments, addictions, etc. and lives in separation and suffering.
A Deluded radiates painfull emotional and mental vibrations. He lives in separation and darkness.
A buddha radiates divine light, peace, love and harmony and lives in unity with all life. He is Brahman itself.