Seems like you could probably get more efficient by using a nuclear reactor to power an ion drive? Also wouldn't need to cool fuel down to cryogenic temperatures.
At 1 AU from the Sun, and possibly all the way to Mars, advanced photovoltaics may very well be better than a nuclear reactor for powering ion engines: It has very high system-level power/weight ratio (in lab around 300 W/kg, currently in operation around 150-200 W/kg), possibly could even power an ion engine without heavy power conditioning equipment ("direct drive electric thruster"), and also scales down for smaller probes. So for a trip to Pluto, a reactor would be useful, for a trip to Mars, it's hardly necessary.
Ion drives don't scale up in thrust fast enough for it to be worthwhile for manned missions.
I think that's basically what the VASIMIR engine is.
For a trip to Mars the time it takes an ion drive rocket to reach cruising speed isn't negligable compared to the overall flight time. And missing out on the Oberth effect is fairly significant. If this were a flight to, say, Jupiter though electric drives all the way.