The two above posts that advocate connecting in China are bad options. Whenever possible avoid connecting to domestic Chinese flights because of the hassle of their airports, the bad lounges and the possibility for delays.
The UA flight from SFO lands you directly in Chengdu after hopefully hours of sleep in their flatbeds (exact same seat as CA and LH). I prefer it that way rather than transiting in Seoul or Tokyo. HK is too far out of the way to make it worthwhile and also subject to massive delays.