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A nodehandle is a RAII (resource acquisition is initialization) type according to source: ros.org. RAII is a C++ term meaning that resources are cleanup up afterwards when the object goes out of scope. In rospy the nodehandle-like class is the rospy module itself. Both solutions seems more idiomatic to their languages, but that is a guess on my part.
Private node handles are a way to access names private to the ROS node, which would create ambiguous names in some situations otherwise. They are a nodehandle to a different namespace. AccessingPrivateNamesWithNodeHandle. I have not used them myself.
underscores at the end of a member variable are a C++ convention. As with all C++ things, this is not universal and will differ in different organisations.

A nodehandle is a RAII (resource acquisition is initialization) type according to ~~source: ros.org~~ros.org - NodeHandles. RAII is a C++ term meaning that resources are cleanup up afterwards when the object goes out of scope. In rospy the nodehandle-like class is the rospy module itself. Both solutions seems more idiomatic to their languages, but that is a guess on my part.
Private node handles are a way to access names private to the ROS node, which would create ambiguous names in some situations otherwise. They are a nodehandle to a different namespace. ros.org - AccessingPrivateNamesWithNodeHandle. I have not used them myself.
underscores at the end of a member variable are a C++ convention. As with all C++ things, this is not universal and will differ in different ~~organisations.~~ organisations.