There's no fixed rule - .com and .cn are equally popular in China. From the speculator point of view, .com is a couple times cheaper to bet on.
If you can get a good name, the higher fee for .cn would be worth it, except, if you can find one, chances that it has little value since good .cn domains are registered by the Chinese themselves.
A word of caution for chinese domains, what might be a good name in the USA is often not the case for China. For example, names like pay.com, buy.com might be useless in the chinese context since most people use the internet to find phone numbers and as a product directory/yellow pages, and not to directly buy goods. The situation might change in the future, but it won't happen in another 5 years.
Popular chinese names include names of types of businesses, product categories, etc.