Guangdong province (over 110 million) might be the most populated Province in Mainland China. GuangDong Province is the Neighbour of Hong Kong SAR.
The argument that more people with access to Shanghai Disneyland than Hong Kong Disneyland is definitely NOT TRUE.
Hong Kong and Shanghai are 2 different Markets. Hong Kong serves Southern China or Greater PRD Region (Greater Pearl River Delta Region), Shanghai serves North Eastern China or YRD (Yangtze River Delta Region). PRD and YRD are 2 distinctly different markets in China.
Weatherwise, Hong Kong is more suitable as a base for theme parks with all year round operations as Hong Kong is subtropical climate. Shanghai is too cold to stay outdoors in Winter.
For Air Travel, Hong Kong is the Mega Hub in Asia.
The following is the 2006 chart for top 10 Airport by International passengers (according to Airport Council International):
1. London (Heathrow)
5. Hong Kong
8. London (Gatwick)
A Regional Express Train (China's own version of Bullet Train) is under planning in Hong Kong. Hong Kong Government has already announced that a dedicated line would be adopted for the Regional Express Train service. It will link up Hong Kong with GuangZhou and the terminal will be built at West Kowloon.