I ordered but haven't received the Zildjian A custom pack. I want to sell one of the crashes. Cause I need the money and I don't have space for both crashes.

Which should I sell and which should I keep?

I play rock music from soft to hard rock. mostly harder though. I know the bigger cymbals are darker and have longer sustain.

I would like to sell the 18" because it's worth more I believe, and it takes up more space (I have a small room). But if the 18" is considerably better than I don't want to sell it.

I would try out the cymbals and decide for myself but I don't want to use the cymbal I'm going to sell.
To be honest, this is all down to personal taste. Personally, I'd keep the 18", since I prefer bigger crash cymbals (I play an 18" and a 20"), however it also depends on the sizes of the other cymbals in the set.

I'd wait until you get the pack, and then decide after you've heard and made decisions on both cymbals.