The problem with systems like ion or VASIMR is that to generate an appreciable delta-v with a manned spacecraft, you need a lot of electricity on the order of megawatts. That leave just two choices: 1) monstrous solar arrays or 2) nuclear power.

Building megawatt sized solar arrays is certainly possible but they'd be enormous. Not only would deploying them be "fun" but I wonder if vibrations would be a problem during mid-course corrections and other maneuvers. There's also the issue of reentry where you'd likely have to dump the arrays before coming in. That makes reuse problematic.

We all know the problems with nuclear power aren't limited to the technical issues. No one has ever flown a megawatt sized reactor in space so there would be some development required. Then there are the Greens....

It might be possible to fly the entire mission in a reasonable timeframe without needing exotic propulsion. In the long term, I'd love to see something like VASIMR used but if the goal is to launch within the next 5-10 years, it just doesn't seem likely the technology will be ready in time, especially given the budget constraints.

Other issues like life support will be difficult enough without adding the complexity of exotic propulsion and electrical power systems. KISS is a very good idea for space systems. The famous aeronautical engineer and test pilot Geoffery DeHavilland noted long ago that the goal was to "simplificate and build in lightness." Bill Lear's guidance to his engineers during the development of the Lear Jet was "if you don't need it, leave it off. Not only do you save the weight of that system, you save the weight of the backup."