The additional sliders makes it more than a software upgrade as well as a host of other physical features including the Hard drive and the movement of the pads to the left side where they should have been all along.
% effects per style is nice as well as the articulation buttons. The PSR900 is far closer to the Tyros 2 then the Tyros 3.

There's where you can really see what Yamaha has improved upon. They did a good job and I'm looking forward it.
Its all about Styles or why not just buy a Fantom, or M3,or Motif. If you cannot get thousands of styles and a huge base of support in that regard. having an Arranger with great sounds is just another very very expensive 61 or 76 unweighted key synth