Aerojet cite Solar Electric Propulsion as an enabler for an Exploration Gateway

NASASpaceFlight.com



This 2012 article seems to be on the right track.  Ion propulsion is much more fuel efficient.  It takes longer to arrive to a destination, but that doesn't matter if the idea is to place infrastructure into place.  This infrastructure will save money because it will allow reuse of expensive modules.  Furthermore, you don't have to drag a whole lot of hardware with you on each trip because one is there already and waiting for you.

Some of the discussion sadly repeats the same pattern of Apollo.  Send up a bunch of hardware that only goes on a long trip in order to be sent back.  All of this requires a lot of mass, which causes you to have use the big rockets.

The best use of the big rockets is to send up a butt load of hardware that is then forwarded to the LaGrange point by the use of ion thrusters.  For example, a lander, empty fuel tanks, and a habitat could be placed a EML-1/2 on a single launch of an SLS.  The next mission could send a fuel for multiple landings. If there would be another SLS, send mining equipment down to the surface to gather up LOX.  This would be a big help in saving mass that has to be delivered to the station.

Finally, a mission which may not even need an SLS could send a crew.