Which card to buy in Hong Kong, Octopus or the Tourist Day Pass?

Hi, I am confused, my family and I (4) will be staying in HK for 6 days. Upon arrival I wish to board service A21 to Nathan Hotel and afterwhich I wish to travel within HK during my stay by taking the MTR and the buses too. So which card should I purchase? My places to cover would be The Peak, Disneyland, Ocean Park, Stars Avenue etc. Please advise, thank you.