I haven't flown CO or US, but here is my comparison between AC and UA C (new seats in UA) that I posted in a similar thread a few months back.

AC vs. UA International Business

Both offer lie-flat seats, but AC offers the pods which offer a great deal more privacy. That said, the AC pods are not as wide as UA's C seat, so I actually recall sleeping better on UA.

Food is roughly comparable for both although Air Canada has recently enhanced their dining options with new menu items and four entree choices (up from three previously).

For in-flight entertainment I prefer UA's larger screen and greater non-movie options, but in the end, you get on-demand audio video on both.

One major advantage for AC are their lounges, which are far superior to the UA RCC's. The Maple Leaf Lounge has free liquor (spirits, wine, beer and local brew on tap all self serve), plenty of soft drinks, better food including soups, salads, pasta and cookies, better atmosphere and better features including showers at their main hubs. The Air Canadaarrivals lounge at LHR is also available to anyone travelling in C, even those who've upgraded or are on reward tickets.

Generally, I would argue that transfers at UA hubs tend to be more seamless than at AC's hubs in Canada, especially when transiting from Europe to US. The baggage tended to come off much quicker when I was transferring through SFO or LAX coming in from LHR, then at YYZ (Toronto) or YVR (Vancouver).

SO, to summate AC vs. UA

Seat: AC has more privacy but UA's is more comfortable for sleeping
IFE: UA wins because of the bigger screen, more non-movie options and because you can watch it while taxiing, taking off and landing (AC's has to be stowed)
Food: Tie, both are comparable
Lounges: AC hands down
Service: Tie, both are comparable
Transfers: UA, from my experience my baggage just came quicker when flying UA then with AC.