Hu-Wen inherited the warm relationship with the US from Jiang, after Jiang skillfully prevented the relationship going down the toilet after Tiananmen. There is no surprise that Hu-Wen tried to maintain that warmness by playing nice, after all China also just entered WTO. Xi on the other hand, inherited worsening US-China relationship from the Hu-Wen administration, due to China meteoric rise after the US financial collapse in 2008. So no surprise that Xi is playing hardball to defend Chinese interest.