it's a nice subject. Depends on many factors I would say. First question would be, in which stage of the plant? If it's mid to end of flowering, then I would personally go with lower temp and ventilation. That will actually help and it's a trick to get your plants frostier. I am watering the last 2 weeks with ice cold water full of ice cubes and it gives good results. The plant will think it's winter and it will produce more trychs for hit protection.
Then if you are in vegetation...still you'll be in a schedule of 18 light with 6 dark, so I think I would personally opt for the ventilation. Another question...what is the temp when it's dark and with the ventilation on? and what's the humidity like?
I am a closet grower myself and have some humidity issues as well. During the dark period I am using an electric heather which is keeping the temp constantly to 24 C and my humidity sometimes goes quite high, around 70 % but usually it's about 55-60 and the plants are doing great so far. As per ventilation I am using a normal fan scheduled separately both for light and dark program. During the light I am not using it constantly cause I am using some CO2 pills and I need no air movement while they are acting. Hope this helps
