From the Hasselblad Lens Catalog:

"Makro-Planar CF f/4 120mm:

Te designation "Makro" reveals that this lens possses an optimum image correction at close range. It is a lens to be recommended when maximum corner-to-corner quality is demanded in close-up work, especially when photographing flat copy, where high contrast is also a requirement.
This lens consists of 6 elements in four groups arranged around almost symmetrically to the iris diaphram. The Makro-Planar lens perfroms best in the region of slightly reduced imaging of the subject.
The shortest focusing distance of 0.8 m (reproduction ratio of 1:4.5) can be set using the helical focusing mount. By inserting the Hasselblad extension tube 32 the macro range can be increased to 1:2."

This lens is "optimized" that is, designed with a specific parameter in mind: Close-up photography. It *will* focus to infinity, and I *doubt* (a bunch!!) that Zeiss would allow it to go that far if it was a significant "failure" at that distance.

The 135 is a different animal:

"Makro-Planar CF f/5.6 135mm:

The 135 Makro-Planar f/5.6 lens has no focusing ring and is designed for use with a bellows extension or a variable extension tube.
With the bellows, the focusing range of the lens is contiuous from infinity down to 1:1; lifesize magnification.
Like the 120 mm Makro-Planar lens the 135 mm is optimally corrected for close-up work, but it is also an excellent lens for general photography where maximum image quality and freedom from distortion are required."

So the major difference in terms of use is that the 120 mm will stand alone, and can be "accessorized" with tubes and/or bellows; the 135 mm MUST be used with a bellows or "Variable" extension tube - both of which are not inexpensive (remember that "Hasselblad" in Swedish, like "Zeiss" in German, translates to "Hoo, boy!! - *expensive*), and are somewhat heavy and a trifle clumsy.

Can the run-of-the-mill 80mm Planar be used "satisfactorily" with bellows, tubes. or "Proxar" close-up Lenses? Uh... it depends on the individual's requirements - but it is "satifactory" enough for me.