These two indicators are essentially equivalent in use and interpretation, but one can be easier to use than the other, depending on the data in the chart. Here are two examples, which should work without a subscription.

PPO better than MACD:
http://stockcharts.com/h-sc/ui?s=$CPC:$BPSPX&p=D&yr=2&m…

MACD better than PPO:
http://stockcharts.com/h-sc/ui?s=$BPSPX:$VIX&p=D&yr=2&m…

Discussion:

MACD:
http://stockcharts.com/school/doku.php?id=chart_school:technical_in…

PPO (used much the same as MACD, so this just explain the minor differences):
http://stockcharts.com/school/doku.php?id=chart_school:technical_in…