I think this highly depends on the sort of product you're selling. For example, you're not very likely to buy more than one product in a computer store. If, one the other hand, you're an online supermarket, i would recommend 'Continue shopping'. However, if the preferred option is 'continue shopping', you should think about why you want to show the message anyway. Maybe there is a more elegant way, like putting the basket in one of the corners and let it highlight the moment you add a product to it. This way the shop experience isn't disturbed.