Datacenter USCA_6 offers the best overall network capacity and stability. All China-bound traffic is sent to the CN2 GIA network (this includes China Unicom and China Mobile routes). There is no direct peering with China Mobile on the inbound; therefore China Mobile latency is increased.
Datacenter USCA_9 offers direct China Mobile connectivity on the ingress. However, the outbound routing is different: there is no direct local peering in LAX; instead, all outbound traffic (including non-China traffic) is sent directly to China Telecom. This results in better routing for some China destinations which are not normally recognized as located in China, for example, some universities.