IMO,
18/6 is better becaue you save 6 hours of light...

24/0 will work out fine...

BTW: I think that the options should be 18/6 and 24/0...
Although the 24/7 is understood that the lights will always be ON, the nomenclature we've come to use is to place ON hours first and OFF hours second... so, lights ON all the time would be 24/0.

But, dude! Everything's fine... I'm just nitpicking... maybe clearing that up... maybe a noob might get confused...

Anyways, 24/7 = 24/0...