This is probably the most important question to ask yourself. Of all the mistakes made when choosing a fireplace, or home decorating in general, is mixing and matching traditional with contemporary.
Of course, some homes do a wonderful job of incorporating both types of designs, however they do this on a room to room basis. Meaning, if your living area is traditionally styled - your fireplace should be the same.
Most fireplace designs lean towards being traditional, but whichever you decide it's best to ensure that your living area's decor follows suit.