But then again... I think they're hurting for money; SLS is proving much more expensive than planned. And they have a lot of project managers who remember the Shuttle Era when it cost upwards of 600million per mission to launch the thing. These guys are super keen to avoid a repeat of this situation again if at all possible. SLS is already going to cost around the same per mission due to its reduced launch rate. The commercial-crew program was supposed to provide 'cheap' access to LEO; If NASA is forced to buy Delta4 Heavies for each CST100 mission, the cost is gonna be almost as much as shuttle/SLS!
So if NASA were smart and daring, they'd choose SpaceX and SierraNevada and save mountains of cash... but NASA being NASA, they'll probably choose Boeing and be forced into paying through the nose for launches for another 30 years.