Let's just assume for a second they are both the same price, or one
or the other is slightly lower than the other.
The Canon Rebel would be a better choice, as it offers you the
ability to take photos in RAW OR Jpg formats. The Sigmas only
allow you to capture images in RAW mode. Every single photo you
take will have to be run through the proprietary Sigma software
that comes with the camera.
Though Sigma's own RAW conversion software is very, very good - the
format is also supported by Adobe RAW in PhotoShop CS.
You'll have to adjust various things
to each image before resaving them as a Tiff or Jpg for use in
whatever imaging program you use. In other words, while the Foveon
sensor the Sigmas use is interesting and shows great promise for
future cameras, it is not appropriate for "general consumer" use,
including yearbooks.
Actually, I believe the SD9 might make an excellent yearbook camera
since it operates very well in a controlled lighting environment.
As for which might be better - that really depends on what you need
in a camera. The DR has appreciably better high ISO capabilities
and the SD9 battery situation is sort of a pain in the back side,
but the body build is better on the Sigma and image quality up to
ISO 400 is very similar with the SD9 having better pixel level
sharpness. Of course there is a much wider selection of lenses for
the DR which is a major plus.
Lin