I could make a case for either place based on your preferences.

Tokyo is the more interesting city in terms of specific tourist sites and architecture. Also, the culture there is more focussed on a single ethnic background. Moreover, the overall Japanese culture is likely to strike a Westerner as being far more polite than you'll find in Hong Kong.

Hong Kong is the more interesting city from just about every other perspective I can think of. Visually, it is an amazing place, with the central part pf the city sitting on opposite sides of a magnificent and incredibly busy harbor. In Hong Kong, all of your senses are involved with every step. Hong Kong is a ***much*** easier city for a tourist than Tokyo. The transportation system is the best and simplest in the world, and you'll see 10x as much English on signs as you will in Tokyo. And then there's the question of finding an address: simple in HK, but you really don't want to even hear about Tokyo.

To toss something else in the mix, one of the most fascinating cities in east Asia is Kyoto, with an almost overwhelming mix of culture, architecture, and history. While Kyoto is certainly a large city, it isn't nearly as overwhelming as either Tokyo or Hong Kong. Kyoto is about 70 mins by train from Kansai Airport (KIX).

Summary of my opinion: If you love cities, Hong Kong is easily the 1st choice; it's the best city in the world to visit. If you enjoy cultural/historical stuff and could do without the biggest crowds in cities, Kyoto is the place. If your best friend lives in Tokyo, that might be a reason to go there.