For the OP--
This thread has some great information.
May I just say that these mics are made to not influence the sound. Therefore either omni will present your piano as it is. When you get to the top level performance of great mics like these, you are paying for neutrality.
It is the placement and positioning of the omni mics that will vary the tone of the recording--capturing either more or less treble and bass.
Also the main influence on the sound of piano recordings is the room.