You can't really compare them unless you have played both for a fair amount of time.
I love the Audya. I haven't even listened to most of the T4 demos, because the only way I would want to buy one would be after I tried it in person. What little I've heard seems to be a little better than previous incarnations.
I fear Yamaha has not yet improved the drums enough. I could be wrong. They SAY they have improved the vocal harmonizer but I haven't heard it, much less tried it. To make a comparison between a board with which I've spent hundreds of hours and one I've only heard on a few demos and that is not even available yet would be fruitless and unfair to both.
Bear in mind that I want to sound like a live small combo, not a full-fledged music production, at least most of the time.
I have found my Audya to be almost flawless and the Operating System to be easy to use and more logical than the Yamaha. Maybe not as logical as Roland, but close. And it is miles ahead of Roland as far as PC connectivity and outboard storage.
The one thing I would welcome most would be the OTS system of either Roland or Yamaha, with all the great lead sound presets they come up with.
DonM