It depends...are you satisfied with a "get by" instrument, that meets your needs...or finally decide that you want a "players" instrument, that the company designed to be it's top model..

It would be such an easy decision for me...S950 is an instrument that I don't want to play..ever ..and the Tyros4 the first Tyros model that I would be interested in using..
Good luck, finding a limited edition model (and you will pay through the nose)
What I would do....Get the best Yamaha has to offer..features are better..Tyros5..and it would have to be 76 keys...if you are going to be a player...get a player keyboard