I find Canon's approach of building two sensor sizes may be a roadblock for Rebel owners who wish to move up market since their EF-S lenses will not work on the full-frame 5D or 1D models.
This only applies if you bought the EF-s: 3rd party 'digital-only' lenses from Sigma, Tokina, or Tamron (which are equally excellent in most cases) have the full-frame mount which mean they also fit the full frame albeit they might vignette which are no different than the Dx lenses, but then you can still crop the resulting image in Photoshop
BTW Canon's has three
sensors sizes (not two) for example the 1D MrkII/N @ 1.3x (APS-H) is half way between the APS-C and the full frame
-> This format can benefit from both, like the cropped advantage of a telephoto or the slighly better dynamic range of the full-frame...