I am torn between Claude and GPT. Though it was recently brought to my attention that I use LLMs in an old-fashioned way [1]. I will say that based on my usages, both models seem very comparable in terms of accuracy and quality. Sometimes one might do things a bit different, but both tend to be more similar than different.

When I am using an LLM for JS, I can't really tell the difference between the two. For C#, I think GPT might produce slightly better quality code, but Claude produces code that seems more modern. I also feel like Claude makes slightly more minor mistakes like forgetting to negate a boolean conditional check.

With Swift, I have found both models to be surprisingly awful. I am not sure if it is because of some of the more recent changes with Swift versions greater than 6.0, but both seem to produce wild results for me.

[1] I do not use Codex CLI nor Claude Code nor any IDE plug-ins. I just type questions into the web app and rarely copy/paste anything back and forth.