CO flies swarms of ERJs all over Mexico, and densely-configured 757-200s across the pond. They fly densely-configured 738's on most transcons. The product differences vs. United are about as extreme as you could have between two airlines.
If UA bought CO and upgraded CO's fleet interiors to UA standards, ^
One exception is probably CO's service to/from Hawaii. I was on a different message board yesterday--geared more towards leisure flyers--and one CO flyer made a pretty good case for flying CO vs. UA on flights to Hawaii. After doing some research here and on SeatGuru, I might have to agree. CO's 767s to/from HNL have better in-flight amenities than UA's two-class 757/767/777 domestic configurations. Their BusinessFirst seats are of the almost-lie-flat variety. And CO has personal IFE systems in both cabins. If I'm reading SeatGuru correctly, UA's domestic configuration of these planes do not have personal IFE systems and the first class seats only have 38" pitch. Of course, UA's big advantage is E+. But I'd feel strange to be in a UA 777 w/o my own video screen, airshow and 9 channels of entertainment on the longer flights. I guess I'm spoiled. LOL!