Is chemical stripping as good as bead-blasting to prepare a steel frame for repainting? I have a rather nice old hand-built 531 frame and fork that I'd like to have repainted. Some companies offer bead-blasting to prepare it, but others offer chemical stripping.

Is this as good as bead-blasting for steel; does it offer any advantages?
Both processes serve to remove paint coat from your frame. The only difference is that bead blasting may (or may not, depending on the medium used) scratch/damage the metal surface of your frame.

If you don't like that possibility, then you can opt for chemical stripping (especially if they'll treat the frame after for effective coat adhesion), but I'd consult with the establishment first on how your steel frame would react to the chemicals.

With that said, the link you gave also advertised giving treatment to your frame to prevent under-coat corrosion, so that's a nice touch for me.