I finally been able to test both the px and the piano nord 4
Clavia library are great, especially piano (of course), strings, brass and choirs, really close to a the real and ones, but px is astonishing, what a spatiality in sound and so clear and defined.
Maybe not all sample are so useful and great (some piano sample did not sound at all like a piano) but overall is great.
I preferred px over grand piano because of its synth features, when playing nord piano I miss having a filter, pitch and mod wheel etc etc but have to say that clavia is great in offering everything without menu while px requires deeper learning curve...
Finally, the keybed of the px is horrible... The xl is similar to hp keybed from clavia or the hammer graded from Korg?