The original Nord Stage is still an excellent instrument and commands high prices on the used market. There's no reason to throw the old one away and few reasons to buy the new one, because the old one does pretty much everything you need it to do.

We're talking about very mature technology. The difference between Nord's 100MB piano sample sets and a 20GB mega-multisample is extremely subtle. The synth and effects engine on the Nord Stage sounds fantastic. You could add a ton of sample memory and processing power, but the sonic benefits would be absolutely marginal.