Both toolkits are actively maintained, and by now more or less equal in features and quality. There are only few, rather unimportant differences.
Still, I’d recommend PySide for Python 2. It has a more reasonable API, mainly it doesn’t expose Qt types, which have a direct equivalent in Python (e.g. QString, QList, etc.) or which are completely superfluous due to Python’s dynamic nature, like QVariant. This avoids many tedious conversions to and from Qt types, and thus eases programming and avoids many errors.
PyQt also supports this modern API, and uses it by default for Python 3, but not for Python 2 to maintain backwards compatibility.