Eobd2.fr here lists some basic facts and differences between to lead you make a better choice.
Cheap Piwis cable:
The cheap piwis cable is the handy Porsche diagnostic cable that DIY users ever need. It is compatible with free software.
PIWIS Cable achieves:
Read faults codes
Log and reset codes/CEL
Detect actual values
Oil service reset
Cruise control activation
Service reminder resets
Free download PIWIS cable software:
A Porsche Piwis tester 2 is a comprehensive diagnostic system for experienced Porsche owners, technicians and mechanic.
PIWIS tester 2 does everything cheap Piwis cable does, including allowing you to make changes to the DME. The PST2 is the PIWIS predecessor, but it can not program cars after MY2004 and has limited programming for Cayenne.
PIWIS can do more than 50 times what the Piwis cable software can do - and the Piwis cable software can easily do 20 times what a standard code reader can do.
If you are a DIYer and want to realize basic diagnostic function, you may consider Piwis cable software. Otherwise, choose Porsche PIWIS Tester 2 (PT2) instead.