This fairly recent thread elaborates on WTA vs PPSC:
http://www.webdiplomacy.net/forum.php?
threadID=1201652#1201652
Maybe JaimeR will earn some more +1s for his statistical analysis.
Some commentary from none other than the creator of Diplomacy, Allan Calhamer, who argues against PPSC:
http://www.diplomacy-archive.com/resources/calhamer/objectives.htm
See also the "Calhamer Point Count" scoring system for tournaments:
"CALHAMER POINT COUNT (1) [MB:Jun80]
A rating or scoring system named after the game's inventor, giving 1 point for a win, and 1/N points for a N-way draw. It reflects the "win-only" philosophy. In the "Averaged C.P.C", this score is divided by the total number of games, and in the "weighted version", corrections are made for the relative success of the 7 countries. Present custodian is Doug Hollingsworth. See Rating Systems (KW). "
I saw this after my scoring system proposal in the thread listed above. They are pretty much identical except that I multiplied everything times 420 so that scoring is done in whole numbers rather than fractions or decimals.