Cgiles, I watched a basketball game and part of a golf tournament. Kept falling asleep, but still better than watching the Oscars.
Zuki, for quite some time I used arrangers as the "top" keyboard and a DX7, later other synths on bottom. In addition I was midied out to drum machine, piano module and tone module.
I used a midi foot controller to change patches on a pitch shifter for vocal harmony. Thousands of dollars invested. Now it's all included in the arranger. PSR900 has all that for maybe 1/5th the price, and 1/10th the weight.
If you like the sound of the Yamaha drums and bass, and can live with the vocal harmonizer, it's very simple--it's all in the S900!
BTW, I have one for sale. Sorry, couldn't resist.
DonM