Hi Frank
The PA800 is considerably more expensive then the S900 and so they cannot be compared like for like, however both are good at their price points.
I you are looking at the PA800 (Make sure you demo it with the latest OS 1.5) also take a look at the Ketron SD 5 as another option.
In all cases you will need to go for 3rd party styles, (Or create your own) as Arrangers are general purpose and so having a limited range of all different style categories. (Yamaha probably has the most 3rd party styles available)
Hope this helps

Bill