Boeing CST100 is the most conventional of all the 3 designs. It's probably the 'safest' bet but also the most expensive to operate. This is because it still requires an Atlas or Delta rocket to launch. Atlas is likely out because its russian-sourced motors are currently out of fashion. That leaves Delta; which is the most expensive of the currently available launchers. Also, CST100 uses a traditional 'SplashDown' recovery that requires a flotilla of helicopter-capable naval ships to be on standby for every mission. This is not cheap. That said, I'm sure it will work as-designed and will be fairly reliable. But all this comes at a cost; and IMO if NASA are smart, they can get more functionality for less money from the other providers.
Lets see: SpaceX has the advantage of using their own launchers. These were already much cheaper than Atlas and Delta, but soon we're promised that they'll be reusable. This can slash the cost by a further factor of 10 again. Such possibilities are hard to ignore. Furthermore, the Dragon2 capsule does not require a naval recovery and can land on a simple slab of concrete anywhere you'd like. There are other fringe benefits too... Dragon2 is re-usable, you could potentially just refuel it, put it back on another rocket and launch it again within a few days. Also, Dragon2 uses a novel pusher-type launch-escape-system that's integrated into the capsule instead of being jettissoned on each launch. These things add up to considerable savings. NASA would be foolish not to choose SpaceX as one of their 2 choices... Price-wise, SpaceX is probably the cheapest of the 3, however the risks are fairly high; there's a lot of new tech that needs to be proven. But the potential rewards are so high and SpaceX's track-record of acheiving their promises is so stellar, that NASA surely must give them a shot.
That leaves SierraNevada. They're kinda the wildcard in this competition. They have this wacky spaceplane that looks like it came from the 60s. If you ask any astronauts which one they'd pick, I bet they'd all say SierraNevada. Why? Astronauts are often test-pilots. Test pilots like to fly things. This is the only one that can be 'piloted' in the traditional sense of the word (with wings and surfaces). It has several advantages: Easy and fast recovery; just land on any runway. Reusability; no need to build a new one each time. Pusher-style LES. Also, it just looks plain cool. And it has one big disadvantage: It's designed to launch on the currently-unfashionable AtlasV rocket. If SierraNevada can convert this thing to launch on a Delta or a Falcon, I think they'll have a good enough price advantage over CST100 to make them the 2nd-choice after SpaceX.
Having said all this... space hardware gets built on the whims of politicians. Even with the best planning, there's still a good chance that some politician will point at the pork-barrel and fuck it all up.