I wish I had a piano to record, I could give you a bit of a test. I've got the M5s, along with CAD 1200xls, AKG P170 and Lauten LA120s. They all sound different, although the CAD and AKG mics sound somewhat similar in response. The M5s are a bit leaner sounding, and the Lautens are a bit fuller on the bottom based on my testing with acoustic guitar.
I posted some samples with the AKG P170, M5, LA120, SM57 and Rode NT1 LDC in this thread. You might give it a listen. The SM57, AKG and M5 mics are the same price ($100), the Lautens are about double that, and the NT1 is about $260. I recorded a short track with all 5 mics, and in the track, I flipped through the group twice.
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I think the Rodes would do a good job, the won't be quite as full on the bottom end, but you could always give a bit of boost in the low end if you need it. I think in a mix with acoustic guitar, it sits in the mix a bit easier. If you're just doing the acoustic guitar by itself, the Lauten fills out the sound a bit better.