The two spacecraft are meant to do two completely different things. Orion was designed for deep space exploration, Dragon is not.
"Red Dragon is a modified SpaceX Dragon capsule for low-cost Mars lander missions using Falcon Heavy rockets. Plans call for a sample return rover to be delivered to the Martian surface while also testing techniques to enter the Martian atmosphere with equipment a human crew could eventually use."
[1]: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2012001...