I have evaluated ChatGPT, Perplexity, and Claude for Wolfram Language coding. Claude Code is the clear winner. I can rough out solutions in my domain (signal processing) within minutes. Using precise prompts, I receive WL code that runs successfully on the first pass 80-90% of the time. From this starting point, I iteratively add features and requirements, ultimately producing 1,000-3,000 lines of production-quality WL. I also use the Notebook analysis function as a linting tool to identify and avoid issues in the raw code provided by Claude.
Regarding your question, I have used Notebook Assistant for more complex language-domain problems, particularly for code optimization. Interestingly, I find Claude exceptionally adept at decoding the sometimes cryptic error messages from the Mathematica interface. Claude can ingest error messages and provide code fixes and simplifications. In my opinion, Notebook Assistant is less useful for addressing the detailed bugs that inevitably appear in code. There is a difference in repairing syntactic errors (Notebook Assistant) verses structural errors where the LLMs like Claude can contribute. One area I am exploring is using Notebook Assistant to generate test code for Claude-produced implementations. This approach seems promising, particularly for solo WL users who lack in-house WL experts for code reviews. I caveat my characterization of the Notebook Assistant a bit because it remains a small part of my evolving workflow.
I plan on testing Claude v. Notebook Assistant in the future, likely using the same prompts in both to compare the results. I think the two models come from different places, so I am not sure if that test will provide much insight as to which is 'better'.
I believe the optimal solution may be a hybrid approach that leverages LLM providers alongside Notebook Assistant for deep, domain-specific training. That said, for example, Claude can be fed explicit domain knowledge using the Projects feature and tuned to produce better first-pass results.
Good luck developing your process!