I feel a bit silly asking this, but I would appreciate clarification.
The Google Placement Tool returns sites that accept Google Ads via AdSense. Google AdPlanner returns sites that accept advertising, but not necessarily through AdSense. Therefore, the sites you find with AdPlanner might require additional work and that's why AdPlanner provides links to the vendors advertising info page.
yep, and there are a few more demographic features, and the output differs because one is simply to find sites, while the others purpose is to do that, but to also devise a media buying plan, including costs and demographic / topic reach.
if you're familiar with the placement tool, looking through the adplanner nav video (6 mins) and you'll see the gist of the other features: www.google.com/support/adplanner/bin/answer.py?hl=en&answer=172381
for others, <2 mins overview video of what it is: www.google.com/support/adplanner/bin/answer.py?answer=114984