The choice of a microphone depends heavily on the acoustics of the room or hall where you will be doing the shooting. If you are thinking of a single mono microphone for recording both performers at the same time, you would have to spend some time finding a position for the microphone that gives an acceptable balance between the two, given that the sounds that a voice and a piano each produces is so different.
Is there a possibility that you could consider separate microphones for the two sound sources and then mix them to a balanced final track in post production? Setting up different microphones for the voice and piano is likely to be much quicker and less tiring for the performers than spending the time to find a workable position for a single microphone. In addition, the types of microphone conventionally used are different for the two sound sources, so the acoustic result from a single microphone is unlikely to be optimal.
Perhaps the best one could imagine is recording in a concert hall (with or without audience), but where there is a good-sounding grand piano and decent hall acoustics. I have made passable last-minute recordings of piano and voice in these conditions with a single co-incident pair of small-diaphragm condenser microphones on a tall stand in front of but above the vocalist, aimed down roughly at the front edge of the piano. I was acutely aware that a carefully-planned multi-microphone recording would have been so much better.
To get back to your question, we would need to know a bit more about the situations you would record in, and also the purpose of the recording (archive, competition/examination entry, broadcast etc), and whether you are including the option of stereo recording.
