In this months Popular Mechanics they have a prototype of an ION engine they say can travel to Mars in 39 days.
I like the Ion engine for unmanned probes or unmanned supply vehicles, but for human flight I would prefer a design that utilizes straight nuclear propulsion or nuclear thermal, with it, unlike Ion propulsion you can boost at 1 gravity for the entire flight except at turn around, this eliminates the hazards associated with weightlessness in space and shortens travel time.