Ion and Other High-Efficiency Drives
Ion engines have now been used to take probes to several asteroids. These engines use their fuel only a very little at a time. The way they shape electric fields to accelerate a stream of ions means they can act only on a miniscule amount of fuel at a time. Though the ions leave at extreme speed, and so gram for gram they provide much more thrust than fuel in chemical rockets, the ion stream is so small today's ion engines provide less thrust than could be provided by a determined bumble bee. That's not an exaggeration. But, they will fire continuously for months, and in the end they will accelerate a spacecraft much more for each gram of fuel than any other engine in use.
That could make them good for getting a cargo ship from low Earth orbit to lunar orbit. The mass saved because less fuel is needed means a much bigger payload can be delivered. Not quite as much as the difference in Isp suggests - the slow, spiraling trajectories used by low-thrust motors take a bit more fuel because they spend far more time breaking away from the pull of gravity near a world. But the portion of a spacecraft that is payload is still several times more than chemical rockets, or even nuclear rockets, can deliver.
However, it would take roughly 6 months for a cargo ship to reach the Moon from Earth orbit, and that could be a damaging bottleneck for the overall project. The early missions are primarily to test prototype robots. Once that test data from the lunar surface is in, engineering teams would have about 2 months before the ship is back and ready for a new payload. (The return trip is faster because it takes less delta V to break orbit, and the ship is much lighter.) Then the engineering teams would have to wait a further 6 months for the new versions to be tested, and again have 2 months to redesign the prototypes and build them so they are ready when the ship gets back. The engineers would either be sitting on their hands or racing to complete new iterations. Since there will be at least one crewed mission in support of all that testing, which couldn't possibly spend six months getting to the Moon, there will also necessarily be at least one large mission using conventional hydrolox (burning hydrogen and oxygen) upper stage engines. So, the cargo ship might as well have next-generation hydrolox engines. After the first tether orbiting the Moon is complete, there would be no further role for an ion-drive ship on the Earth-Moon run, anyway.
For launching from the Moon to deep space destinations like Mars or Venus, low-thrust ion drives could be a good choice. NEXT ion drives require a tenth of the fuel of hydrolox engines for the same delta v, a HiPEP ion drive about a fifteenth (that one is still in development though).
Neumann drives would be even better if they are successfully developed. They employ micro-second pulses of electric arcs to create tiny puffs of super-hot plasma on the surface of a disk of fuel. The hot cloud of atoms expand into the vacuum of space, resulting in them pushing the rocket in the opposite direction. They look promising as long as no issues show up during on-orbit testing of a working model, which should happen in 2018 on board the ISS. They might be available in a decade or less. They could be fueled with magnesium sourced from the Moon, or aluminum collected from scrapped satellites and spent upper stages, or even carbon from the waste produced by a human crew.
The VASIMR drive by Ad Astra is another promising candidate. It too is ready to be tested in space. It uses shaped radio waves to create ions and super-heat them into a plasma that is then magnetically confined so it leaves the nozzle as a narrow jet. It can burn hydrogen, argon, or xenon, and can vary its thrust, unlike the other options.
Each of these systems have their pros and cons, and different situations can favor one over the other. To push large vessels, all of them require the development of big power systems capable of feeding the engines energy at a very high rate. They all need giant solar panel wings, for missions that don't go farther than about the orbit of Mars, and nuclear power plants for missions further out.
These drives come into play in the timeline when asteroid retrieval begins. The Fetcher ships use them.