The biggest difference is focal length: 24mm is much wider, while 45mm is a normal/short-tele perspective by comparison. So the right choice depends mostly on what you shoot.

For architecture, the 24mm is generally the more common choice because the wider field of view makes it easier to fit buildings and interiors into the frame while using shift to control perspective. The 45mm is better when you want a narrower view or less exaggerated wide-angle perspective.

Their tilt/shift ranges are very similar: the 45mm offers about 11mm shift and 8° tilt, while the 24mm offers about 12mm shift and 8.5° tilt. In practice, the creative possibilities are broadly similar; the images differ mainly because of the focal length.

The 24mm costs more mainly because it is a newer design, it’s an L-series lens, and wide-angle tilt-shift optics are more complex and expensive to design and manufacture. The 24mm also allows variable orientation between the tilt and shift mechanisms, which is a feature shared with Canon’s newer TS-E designs.

Maximum aperture usually matters less with tilt-shift lenses, since tilt is often used to increase depth of field and these lenses are commonly stopped down rather than used wide open.