I have the Arca 4x5 and wouldn't trade it for any other camera.

While I haven't used the 6x9 Arca, I know that there's a difficulty using Polaroid on this camera in the vertical orientation. One must raise the rear standard far enough for the back to clear the camera. Of course, one can also raise the front standard so that the two match, but this seems a little disconcerting for such a high end camera.

If I were shooting a lot of 6x9, I'd probably just stick with 6x9 backs on my 4x5.