Crew Dragon is, for now at least, by far the preferred option. Not only has it proven to be reliable, the vehicle also offers the lowest cost due to its reusability and the fact that it flies on the Falcon 9 rocket.
Starliner, by contrast, has several disadvantages. Its Service Module, which houses all of the costly propulsion systems, does not return to Earth after each flight but rather burns up in the atmosphere. Sources said this expendable module costs tens of millions of dollars. There is also the fact that, after its initial run of NASA missions, Starliner will have no rocket following the retirement of the Atlas V vehicle. United Launch Alliance’s Vulcan rocket is an option, but there would be not-inconsiderable costs to human-rate the rocket, and it would be more expensive than a Falcon 9.
Could someone else acquire Starliner? Boeing did begin shopping some of its space assets, including Starliner, a year and a half ago. No sale has been closed however, and one person told Ars that it is because Boeing has been asking too high a price. One potential industry partner with deep pockets, Blue Origin, has instead decided to press on with development of its own in-house crew vehicle.
In a world where SpaceX seeks to retire Dragon in favor of Starship, and in which the space community is not yet comfortable launching humans on Starship, and in which Starliner is actually flying, things could get interesting.
One thing does seem clear. No matter what happens, the cost of human spaceflight into low-Earth orbit during the 2030s is going to be significantly more expensive than the cost in the 2020s, at least until Starship really takes off.