I think no matter what headphones you get, they will compromise on either one or the other. Piano is a really special instrument with a wide range and lots of rich, organic overtones and subtle, interacting acoustic energy transferring between the strings (one note has three strings, did you know that?). Guitar is lovely too, in its simplicity. A truly perfect guitar note starts out rather triangular, and eventually attenuates to the closest one can come to purity, the sine wave.
You really need two sets of headphones.
And on ensembles, don't get me started.