Lens Babies do change the plane of focus, but there are also other aberrations present because the lenses in them are fairly simple optics.
On the other hand, a T/S lens or adapter will allow the photographer to pursue different effects. Shifting the lens will help with perspective. This is usually employed in keeping buildings and trees straight by allowing the photographer to keep the film/sensor plane parallel to the subject. I've also used shifts to keep my reflection out of glass and such by altering the camera position.
Tilting a lens alters the plane of focus. A fellow by the name of Scheimpflug figured out that if you bisect the angle between the film/sensor plane and the subject with the lens plane, everything in the subject plane would be in focus. So imagine your camera is at a 30º angle to a building. If you tilt the lens to 15º, the whole front of the building will be in focus, no matter what aperture or how far away it is.
Here's an example of doing what I described above:
Flickr Link
Conversely, if you tilt the lens away from the plane of your subject, you can really isolate your subject:
Flickr Link.
Both shots were shot around
f8 or
f11, IIRC, with the exact same camera and lens set-up (4x5 w/ Schneider 150mm
f5.6 Symmar Convertible) but feel completely different because of the tilts used.
Lens Babies are interesting tools, but I don't think that they'll give you the creative control that a Tilt Shift Adapter would. The only bum part is that it'll be really hard to get really wide with an APS-C camera body, but you'll still have a new world open up to you.
Good luck!
EDIT: Now that I think about it, the first shot was probably with the Scheider Super Angulon 90mm
f8, but the principle is still the same.