The ORD-HKG flights pretty routinely have UA's new C hard product, which is quite nice. Your other options feature UA's old C, which is a fairly pedestrian product. CA's new C seats (only on the 330s) are rather nice (but not nearly as nice as UA's new product), but their old C seats are about equivalent to the early 1990s UA C product.

Service-wise, CA can be so-so to bad. I've had okay flights because I spoke Chinese, but I've also had really bad flights. If you only speak English, I think their English level is often fairly poor. The food etc. is about as bad as UA, but more geared toward a Chinese palette. The IFE is a joke - they have commercials inserted during the middle of movies that they offer and they usually only offer two or three grainy movies. Most of their movies are in Chinese.

If you care about miles at all, the UA flights will be nice because, with the DEQM promotion, you will earn triple EQMs on the flight.

In summary, if you don't care about miles and speak Chinese and can get a seat on a A330 flight, CA isn't a bad option. Otherwise, I'd go with UA. Try to route through HKG and get on the newly-configured 744 birds.