Lockheed Martin, mostly, develop both he Orion and the CST-100 for NASA. I feel like I'm a bit late at learning here, but are they basically the same designs? The LEO and the Beyond-LEO version. CST-100 as a kind of evolution from the now discontinued an Orion lite project. And with Bigalow involved!? What a small world.
A big difference is that Lockheed Martin has spent a lot of money on radiation management in the Orion vehicle. That is, they have looked at shielding, placement of bulkheads, food and water storage, etc, so as to quantify the level of radiation you would take in the vehicle.
Also, should there be a solar storm, where is the safe spot in the vehicle, and how to orient the vehicle for maximum protection. Face the Service Module into the direction and get everybody huddling in the bottom, to allow fuel in the SM and storage supplies to offer as much protection as possible.
This is an area SpaceX, Boeing, and Sierra Nevada have been able to mostly ignore to date. It is apparently a very complex and expensive task to accomplish.
Beyond that, they really just share the same generic shape, a capsule.
- Larger than CST-100.
- Lands in water (vs. airbags on land for CST-100).
- 21 day endurance for crew of 4 (vs. couple of days, then docked to ISS for power/etc).
- Orion Service Module has significantly more delta V.
- Washroom facilities. (CST-100 is most likely to use diapers).
- Radiation protection.
They might be externally alike (as in design/looks), but they are very different spacecrafts.
CST-100's design is for commercial use, and for use within LEO, whereas Orion is intended for use beyond LEO. This alone involves a lot of differences between the spacecrafts, such as heat shield design (Orion needs a more resistant heat shield for entrances from higher altitudes at higher speeds), modules (Orion will have a service module), etc.
Other notable difference is the splashdown: CST-100 uses airbags, while Orion splashes on the sea (like the Apollo spacecrafts).
The Orion Lite project cancellation gave place for the development of the CST-100 if I am not mistaken.
Also, quote from Wikipedia:
The CST-100 has no Orion heritage, but it is sometimes confused with the earlier and similar Orion-derived Orion Lite proposal that Bigelow was reportedly working on with technical assistance from Lockheed Martin.
It would be interesting to elaborate on all the major differences, I guess there must be a comparison list somewhere between Orion, CST-100 and Dragon v2.