Which film speed is better for night, 400 or 100?

I have a pentax k1000 manual SLR camera. I'm setting up a shoot in a dimly lit parking lot at night or possibly a parking garage. I know it's probably not the best idea, but i really want the darkness as opposed to it being daylight, because it works better with my subject. I don't have a flash, but i plan on using a tripod. My teacher told us that if we're shooting in dark places than it would be better to use 100 speed because it's more sensitive to light it would allow more light in, but i've been looking it up online and everyone has been saying 800 speed is better at night. Now i'm very confused and i'm thinking perhaps i misunderstood my teacher. I only have 400 and 100 black and white, so which would be better?