In going the non-spinning glass route,
what is the primary difference between
using a homebuilt 'AgusTube' as opposed to
Canon's EF Adapter for the XL1?

The one major difference is depth of field. The larger 35mm image projected onto the ground glass (Agus's model) provides for a more shallow depth of field than the 1/3" ccds (1/3" = 8.5mm) of the Xl1. With the EF adaptor, you're just adding a lens (like looking through a window) but with the ground glass, you're actually looking at a much larger, flat surface projection of what's coming through the lens.

another difference is that with the Ef adaptor or any mechanical adaptors, you will have a magnification effect of 7.2X due to the difference of size between 35 mm film and 1/3 " CCD. By using a ground glass as a projection screen like the Mini35 or Agus's version, you preserve all the caracteristics of the lens used.