Thanks for the suggestion. I do not know if I will stay in HK within the whole contract period. It depends on several factors such as employment status. By "troublesome", do you mean the phone company may not let go easily even I pay the penality ($500 or $1000 in the case of Smartone) to terminate the contract earlier?
I asked somebody at a Smartone store about prepaid cards. She said that they only offer prepaid sim cards for phone services, not data services. I am not sure if this is really true.