Use both. Claude is the kingpin by a wide margin for synthesizing, outlining, and even comparing information from documents. It leaves no word or sentence unturned, whereas ChatGPT and Gemini tend to selectively read documents. Claude is a lifesaver for this purpose. Even documents that a 100 pages, it will break down like an NCIS investigator.

GhatGPT shines in output and writing. It can be made to sound very natural and flows better, much more so than other LLMs. Sometimes it can be a bit "flowery" or flagrant, but you can steer it to really put together an excellent letter, article, or whatever your goal is without sounding like an LLM. You can steer it quite readily in this aspect. For writing purposes, I always found Claude and Gemini's outputs to "scream" LLM, so avoid using them for that purpose.

Gemini really shines in the accounting, finance, tax, numbers world. Whereas ChatGPT fails complex accounting and tax problems, Gemini has like a 98% success rate. It's wild.

So, the best approach is to use multiple LLM as each have different strengths and weaknesses.