Both cities give me very profound experiences when I first visited. Tokyo is - for me - the urban experience par excellence, as Pickles says it has many many layers. A fascinating place, but not quite so initial accessable as Hong Kong. One walk along Victoria Harbour at sunset is normally enough to capture most people - the view, the people etc. And if you do get bored of HK, a day (or morning/afternoon/evening) trip to Shenzhen in China might be of interest.

If you are not a confident traveller, HK will be a much easier experience. While the level of spoken English gets worse year by year it is still MUCH easier to communicate, get around and get things done.

Either way you will have a great experience, so don't sweat the decision to much!