If you’re still deciding on a third-party interactive dialogue writing tool (although you can always use the Dialogue System’s full-featured in-Unity editor), Richard Rabil, Jr., did a thorough analysis of several tools in An Evaluation of Software Tools for Interactive Storytelling. Written in 2012, it’s still largely applicable today. The tl;dr in relation to writing and testing dialogue is summarized in this chart from the paper, on a scale from 0=worst to 4=best:
However, the whole paper is definitely worth a read.