What is more predominant, PyObjC or PlistBuddy?
Since /usr/libexec/PlistBuddy ships with OS X and doesn't need any developer tools or third party installation, you should assume that tool is ubiquitous and develop against that if at all possible.

I actually prefer plutil to convert things to XML or JSON and then use a script to make whatever changes are necessary, but your options are not bad ones to use either.