Fran, the Midjay has a totally different OS, and I'm assuming the Audya will have something similar. It's really easy to get around, and I love the registration system.
Drawbacks of the Midjay as an arranger would be the lack of ability to stack sounds on the right hand, unless you create a combination and save it as a new sound. This doesn't bother me, because I never use more than one sound except combination piano/strings, and there is a preset voice for that. I also created a piano/vocal OOH voice as well.
As far as the OS on the older models, it works great, but is totally different from the other companies. It is unique to Ketron and there is a learning curve. In many ways I prefer it to that of Yamaha, and in most ways to Korg. I do like Roland's OS also.
Actually one of the things I like about the old Ketron OS is how easily and quickly you can make changes and save them on the fly.
And you can easily edit and replace all the factory styles with your own if you want to.
DonM

[This message has been edited by DonM (edited 08-23-2008).]