They are both used, however 'passionate' implies much, much more than 'enthusiastic'. An enthusiastic employee takes pleasure in their job, works hard and with dedication. A passionate employee loves his or her job more than his or her family, more than life itself: they lie awake at night dreaming about it, it is their lover, their religion.
Older people tend to use the word 'enthusiastic' because it is an accurate description of a good employee. Nowadays, however, the word 'passionate' is generally used, anything less than that sounds lazy or dull by modem business standards. The word 'passionate' is a gross exaggeration, and it makes me feel slightly sick when somebody uses it to describe their work, but business language has a way of being sickening: you need a strong stomach to succeed in business, as well as plenty of passion.