Just a few differences:
1. J version has 4G memory while US version has only 2G memory.
2. J version is significantly cheaper esp. now the JPY exchange rate is favorable for HKD.
3. J version does not provide Chinese menu while US version has it. Though, it does not affect the reading of Chinese books.
4. Amazon.co.jp does not entertain overseas order for KIndle while you can order the US version Paperwhite from Amazon.com directly. That means you can enjoy direct support service for the US version if you ordered it directly from Amazon.com.
Note that the above is talking about the second generation Kindle Paperwhite released in Oct2013.