If the guy at 711 speaks English, ask him. I have found they are "experts" on this type of thing. If not, I don't think it makes a lot of difference, just make sure the instructions come in English! Sometimes you can find someone in the store to help translate.

On my last visit two weeks ago, I bought a "Peoples" card. HK$98 with a value of $98 plus an extra $30 voucher. Calls and SMS are really cheap. I never used the original value in the card. Will save for next time.

Most other cards are also HK$98, but have differing call values. Last time I was there CSL had the best deal. It expired though, because it was a while before I was back (more than 6 months).

The interesting thing is how easy it is to get the SIM. Not even an application or ID check. Just give them HK$98 and you are ready to go. Most other places (Singapore and Taiwan) seem to be getting stricter on this while HK and the rest of China just sell them everyplace, with lots of different vendors and card types.