The difference between the Bosch pump and the Piersburg pump are:
Piersburg is cheaper (Bosch is $160 and Pburg- $130)
Piersburg is physically smaller (comes with a rubber sleeve to allow it to
fit in place of a larger diameter Bosch pump).
The connectors on the pump are slightly different (Bosch just plugs on, but
Pburg you will have to splice in new connectors- provided with pump).
I think that the quality level of the two pumps is about the same.  So it
comes down to if you feel that saving $30 is worth a little more work on your
part (crimping on new end connectors to the wires).  That is about it.

Just FYI,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
Office: (856)-765-1583
Fax: (856)-327-2242