Hello,
I just tried practising a piece (in a rusty state of practise) first using a "pure" Noire (which just isn't pure, however I tweak/tune it) and then on the Modern U.
The Modern U really is pure and authentic, and as such is a great piano for learning and practising.
I don't think Noire could ever reach that. It is/stays more of a toy, even though it is a fun toy to go nuts with e.g. its sub bass and extreme tonal depths. It inspires other improvisations, more a piano-borders-on-atmospheric-synth. Where (sampled) resonances on the U sound so sweet and lovely, I find them ugly-ish on Noire (where they are modeled).
Cheers and happy authenticity,
HZ