Hong Kong vs Singapore?

Are they really THAT similiar? I'm going on a trip to Hong Kong and Singapore...I tryed to change Singapore to Bangkok. I keep hearing these two places are very similiar, but hong kong is much more lively and Singapore lacks it's own culture. I also hear that neither offer that much as far as major site seeing/historic sites and only offer shopping and food, therefore they can become boring past one weeks stay. Is this true? Will I not a feel a great contrast when I'm in these cities? Also, whats the nightlife like in these cities? thanks!