CST-100's schedule looks interesting
According to Wikipedia
Pad abort test in Feb 2017, uncrewed flight to ISS in April, crewed flight in July. Presumably an in-flight abort test will happen in March.
That sounds like a remarkably short time to go from pad abort to crewed flight. It doesn't give much time for any lessons to be learnt from any minor anomalies between the launches.
My guess is that they've 'got' to be ready by June 2017 to get the NASA gig, but they don't think they'll be ready for pad abort before Feb. And they can pretend this is viable by assuming all those tests will go absolutely like clockwork.
So I guess SpaceX will be launching first, after 'unexpected' delays in Boeing's plans.