1. pgTAP is an extension, pg_yregress is not (although there are plans to make it available as such as well)

2. Allows for organizing and structuring nested tests (providing organizational clarity)

3. Binary encoding testing (useful for new data types)

4. pg_yregress manages Postgres instances and their configuration

pgTAP has functionality that is not yet part of pg_yregress (vast library of helpers)

Ultimately, I believe both of these tools can coexist rather nicely. Would be great to integrate neatly.