As far as original music, I think the arranger keyboards are preferable for creating demos and the workstations are preferable for the final product.

I am crazy about my PSR 740, and looking forward to getting a PSR 2000 any day now. If I have an incredible song idea in my head, an arranger keyboard used with a computer sequencer will give me the quickest approximation of my inspiration. But if I had a record contract, I would want to start closer to scratch and would want a workstation.
For expressing a song idea, there are so many great styles out there that can do a song justice, especially with editing with an external sequencer. And if you want better voices, you can use your arranger with a soft synth like Gigastudio.
But I could see a modern Beethoven grooving on a workstation more than on an arranger.
This is just my impression.

Larry