SpaceX has promise, but Boeing has shown it can deliver.
...eventually, and only after the requisite pork has been spread across a multitude of states and subcontractors to keep the requisite congress-critters happy. :(
Not to knock Boeing's technical prowess, but damn - they do know how to play the game (which explains why they're getting a piece of the contract most likely...)
As a very apt comparison, go back to the days when the F-16 first came out: relatively cheap, by some upstart company (General Dynamics), a revolutionary design, the first 9-G capable fighter, and was an all-around workhorse that could do (within reason) damned near anything you demanded of it. It's still in production today (albeit as a division of Lockheed-Martin), with a design that stands to be around for decades to come. Compare and contrast this with, oh, the F-35/6/whatever that's been nothing but a massive money-sink to date.