It depends on when those 12-18 hours will be.

If it's all during the day, 8am to 10pm kind of thing, then I'd say Miami. Better city for a one day trip than Atlanta.

If it's something like 10pm to 12pm, I'd say Atlanta since you're not going to see much of the city (unless you plan on wandering the streets at 4am) and as far as the airport itself and surrounding hotels, Atlanta is much better than Miami.