Which is the king of mobile: China or US? Where is the biggest opportunity?

The numbers for mobile subscribers and mobile web users in China are mind blowing, but when the enormity of the population is taken into account, how does China compare with the US on mobile?
In the previous column, we compared how the largest Chinese and US companies – specifically banks, retailers and internet companies – were performing and reporting their mobile performance. The conclusion: US companies – particularly the big US retailers – have a lot to learn from their Chinese contemporaries.
In this column we are going to consider if the mobile opportunity in China is larger than in the US. This might help explain why Chinese companies appear to take mobile more seriously than US companies.
When you want to rate and compare the size/potential of the mobile market in a given country there are a number of key metrics to take into account.
These criteria vary a little by business vertical, but for retailers and other companies interested in m-commerce, these following are a good starting point (with some headline stats):

Smartphone sales (China: 2.5x North America).1
Smartphone penetration (US: 72%; China: 58%).2