I've only flown AC C and UA C (new and old) and I must say if you can get the new C on UA, I think its a better choice. I flew AC C twice last year SFO to HKG and did not enjoy either flight. I have flown UA C many times for trips to Europe. While the pod does provide a bit more privacy, its was too narrow to sleep on my back while UA C was not an issue (if you are small, then this would not be an issue). Also due to the pods and the angled orientation of the seats, its very difficult to hold a conversation with a companion. Also the new UA C have seats facing backwards. I found this much more comfortable for sleeping than facing forward.
The amenities are about the same, food mediocre for both. The IFE is better on UA with the larger screen. Also, AC shows these long and annoying commercials before each new selection. They are at least 4 min long and if you end up watching a couple of movies and some TV shows, you can waste 20-30 min of the flight on these commercials (its also the same 3/4 commercials each time - ugh!)
Finally, this last part is not about the cabins but having to fly into Canada before arriving in the US. I had bad experiences with the US immigration agents in Vancouver on both flights back from HKG. The whole process is annoying. You get shuffled along to a separate part of the airport and wait in an uncomfortable room while your checked luggage is transferred to your connecting US flight. This took 30+ min each time. Then both times, immigration agent had the worst attitude and no care for how much time he was taking. Much worse than anything I've experienced when arriving thru SFO, IAD, ORD, EWE or JFK. And coming home, I hate the annoyance of having to transit in another airport when a direct flight is available.