thats really only a question you can answer;
with 24 hours on, you may see comparatively faster growth which could result in seeing pre-flowers in a shorter time than using 18/6, but you will use more power. If you have a smallish light or LED's, that may not be an issue for you, but 24 hours on with a 600-1000W HID could add up quickly.
some say the plants "need" a dark period to perform some functions, and its more "natural" but its really a matter of preference.. auto-flowers are not photoperiod sensitive, so if you're using those, its really about your utility bill.