Either one will be right. I say pick one and run with it. Pay close attention to growth rate then in your next grow try the other cycle and see which you like better. I've run both and had practically identical results. With an 18/6 cycle I save a little cash but more importantly I'm able to keep the room temps a little lower which can be an issue for me in warmer months.
I know growers who have very well documented growth increases running 24/0 over 18/6 though. Growing the same strain (clones from the same mother) over a 2 year period they found that they could cut their veg time by 10 days before reaching the desired height to switch over to 12/12.
I've read articles in magazines, chapters from MJ cultivating literature and numerous forum posts all across the internet making a case for both cycles. There are actually write-ups by reputable breeders that pretty much say to throw those 2 standard cycles out the door completely and fine tune what your strain tells you it wants... I have an article by the breeder DJ Short actually saying that for sativas he recommends the light cycle mimic equitorial photoperiods close to 13/11 during veg!
The bottom line is, neither cycle is "wrong" but one may suit your needs more than the other. Good Luck, hope this helps.