There are some reports by analysts claiming that a CO/UA merger would produce more synergies and revenue than a UA/US merger, and other reports claiming the opposite. It is true that a UA/US merger would create more downsizing in overlapping markets creating more pricing strength. In effect it would be a further shrinking of both operations which would be bad for many employees on both sides. Particularly those in the bottom 25% and those on furlough.
Regardless of which reports you believe with regard to synergies, I think the driving force will be the ease of actually executing a merger in a reasonable amount of time and getting all employees working together toward a common goal, as was the case in the DL/NW merger. An extra $36 million in synergies can easily be negated (and then some) in a service industry by years of labor unrest.
A UA/CO merger would actually create more long term strength due to complimentary route systems, resulting in less downsizing, and more growth opportunities, all of which lead to more job opportunities and job security for employees and furloughees and and more cohesive employee group.
Additionally, since much of the groundwork was already laid by UA & CO in 2008, there is much less "heavy lifting" to do this time around. Even the pilots had a framework agreed to for seniority integration much in line with the gains and security attained by the DL/NW pilots. Notice that CO management has already offered CO pilots the new DL pay rates plus $1, stating that their analysis concluded that the company can attain comfortable profit margins at those rates.
As more information comes out I think you will see things shifting to a favorable UA/CO merger. UA will keep it's options open for a US merger only in the event a CO merger does not consummate. In that case I think UA will move quickly for a US merger this time under their terms. If that occurs, unfortunately for all of us the combined airline would be run into the ground since neither has a management team that can make the airlines work. But IMO, at this point the probability of a UA/US merger is diminishing quickly.