Depends *why* I was recording. If just to flesh out some demos or to explore the sounds I could get, I'd use the mics at hand. Of the two dynamics you have, I'd lean more on the Sennheiser for the bulk of the detailed sound. The e906 is a more honest mic (especially switched in high-cut position), while the 57 adds more of its pretty distinctive mid-humped color. If this is the case, take what you have and start listening really closely to what happens when you change mic positions.
If, OTOH, I wanted to record piano for something the public would hear, I'd look into renting mics, pres, a nice instrument, and a good sounding space.