Is there any downside in purchasing high-speed train tickets in China via the official website, instead of third-party agencies?

I read that high-speed train tickets in China might be purchased online either via the official website or third-party agency such as chinahighlights.com or china-diy-travel.com. Is there any downside in purchasing high-speed train tickets in China via the official website, instead of third-party agencies? I wonder whether purchasing via the official website is always preferable or going through some third-party agency has some upside (e.g., price, ticket availability, etc.).