A pillow top mattress is one where an extra layer of padding is sewn to the top of the mattress, making it look like a pillow. It offers better cushioning and bounce to the mattress, with an adequate level of firmness.
However, a tight top mattress is designed differently. It is a thin layer of fabric that is stretched across the top or the surface of the mattress, giving you a smoother surface to fall asleep on. This mattress gives you a better firmness as compared to pillow top mattresses. Hence, if you are looking for a firm mattress, then the latter is your ideal choice.