On the labor front, Delta and Northwest may also have an advantage due to similar wage levels. All three airlines have recently restructured in Chapter 11 and have cut labor costs to the bone. In either merger scenario, unions would want to recover concessions made during the industry's long slump.
"My assumption is that unions will have to be offered some carrots to go along," said Warlick.
Anti-trust concerns, which have thwarted past industry tie-ups, could cause either deal to stumble, but there may be more issues in a United-Delta deal.
"The opposition in Washington could be fierce due to concerns about upward pressure on fares," said Joe Schwieterman, transportation expert at DePaul University and a former marketing analyst at UAL.
He said government regulators could raise an eyebrow at a United-Delta merger because the two carriers have hubs in close proximity to one another -- United in Denver; Delta in Salt Lake City.
Also, Delta's Cincinnati operation, which competes with United's Chicago hub and Northwest's Detroit operations, would likely be cut in either merger, said MIT's Swelbar.
Fleets, Costs a Wash
Combining Delta's fleet with either carrier would be a headache and have limited short-term advantages, Warlick said.
Both United and Northwest have mixed fleets, which include Airbus and Boeing aircraft, while Delta operates a mostly Boeing fleet along with over 100 MD-88s, which are no longer made.
Meanwhile, the cost savings resulting from either deal would be similar. Goldman Sachs estimates the savings from a Northwest-Delta deal would be $819 million, while a United-Delta merger would generate $891 million in savings.
If a merger between Delta and either Northwest or UAL does come to pass, it would involve two healthy carriers -- a rare event in the U.S. airline industry, where deals typically involve at least one of carriers being in or near bankruptcy.
The carriers' shares, which all soared more than 20 percent Thursday following speculation of pending talks, were down in afternoon trading Friday.