Imho,
24/0 for young clones, at least for me.
24/0 during veg is a different story, photoperiod canna doesn't need an absence of light to shut down its foliage, and if your running low to medium light levels then it might not ever shut down because it can heal itself faster than the light can damage it, in this situation 24/0 usually shows faster growth compared to 18/6.
24/0 with high light levels might send your plant into a droop like mid day depression, mid day depression is a response to drought or an accumulation of sugars in the leaves, at this point you are wasting money on electricity.
My take on this,
If your running low light levels or cfl etc, you could try experimenting with 24/0 or 20/0 if you really wanted to, other than that i wouldn't even bother.