If you are flying circuits at your 'home' airfield, then the elevation of it and therefore the circuit altitude should not take long to get used to. Once the student or whoever has left circuit altitude, then they should be flying a visual picture and not be worrying about whether they are, say 700ft AGL or 900ft AMSL when turning final or whatever. It should all be by reference to a correct visual picture and adjusting the approach as appropriate.
Therefore, my vote is for QNH all the time, and a good pre-arrival brief regarding airfield elevation and circuit altitude before going somewhere new.
This provides an easy transition to instrument flying where altitude information is always presented more obviously than height information (and of course then there is the need to cross check actual altitude with approved procedure altitude as there is no visual picture.)