None. Both mean 'you' in accusative, but one is mute form ('cię') and one is stressed ('ciebie').
Stressed is used if the logical accent in the sentence is more on the receiver of the action than on the action itself. ('Kocham cię/Kocham ciebie' - 'I love you/I love you')
Also if you want to begin the sentence with the pronoun, it must be the stressed form.