S900 is essentially a Tyros 2 with less on it. (Less styles, sounds etc)
The S900 will not have too much effect on Tyros 2 sales, as the Tyros 2 is being replaced by Tyros 3 in a few months. (The S900 will then be to Tyros 3 what the PSR3000 was to Tyros 2)
Hope this helps

Bill