Also check the visual aids thread. I posted a picture of myself wearing the 30. I agree that I rarely see celebrities with damier. Since the popularity of damier is increasing, maybe that will change?
IMO Damier needs structure which the 25 would have over the 30. I prefer Damier pieces that are more stiff as the checkerboard pattern is very straight.
I have a Damier 30. If your stuff fills more than half of it, the shape will hold pretty well. I use this for work and when on vacation, and my cerises speedy (25) for weekends or free time when I don't need to carry so much - so it depends on what you use it for mostly.