Quality of materials and manufacture.  Period.  Clutches are a PITA to
replace, and typically last 80,000-100,000 for me here in hilly Seattle.
"Premium" clutches typically have a diaphragm made of heavier gauge steel
than standard, and are stronger as a result.  Assembly quality is also
important.  Many years ago I bought a lesser quality clutch for my MGB, and
it never fully released, causing disconcerting gear grinding and hard
shifts.  After trying everything else to fix it, I pulled it and found that
the plate had been riveted to the diaphragm off center, and could never
fully retract.  

That said, I've had good service from both LUK and Sachs, and I haven't
actually compared the two side by side, or weighed them.  Maybe I'll do that
next time.

Stuart MacMillan
'84 Westy w/2.1

Original message:

From:    Damon Campbell <damoncampbellvw@YAHOO.COM>
Subject: clutch quality question

What makes one clutch better than another?  For example, bus depot sells a
"Premium LUK Clutch Kit, 228MM, 80-91 Vanagon, the best available"... how is
this different/better/worse than a sachs/oem clutch?