Hi,
I would suggest Unity UI. It make sense because it's safer in the long run, and we support it "officially" via the Ecosystem because it's part of Unity. Currently, Unity UI is way more accessible from PlayMaker than NGUI, however, I have done projects with NGUI and the set of actions and proxies available works very well too.
You know where to find PlayMaker support for Ngui and uGui right?
Bye,
Jean