If you're looking to play "modern music" on an arranger..., without question I'd say buy the Korg PA800. The S900 is a nice arranger (it costs less too), but Yamaha arrangers (with the exception of the DJX line) have been very weak in the "modern style" area. IMO Yammies have always worked better for more traditional styles of music.

The Korg is also going to give you a freakin killer synth engine too--plus the sounds are "Triton Based", where as the S-900's are well.., PSR based (Yamaha to date doesn't go the other way in the sound department with their arrangers). It's not to say the PSR sounds are bad.., they'll be quite good, however the soundset won't be favorable for modern music.

If you can afford the PA-800 I say buy that one and never look back With the synth engine, good effects, modern sounds and styles..., I'd say the PA would fit your needs quite well. To be honest I'd even take a Korg PA-50 over the S-900 if it was "modern music" that I was wanting to do with an arranger.