That is the question.
Both of these Image Generators can keep character consistency, and both are easy to use. So how should you determine which one to use for a project?
Nano Banana is very impressive with image quality, text renders, etc.. ChatGPT produces good output as well, and I personally think ChatGPT's image generator (a hybrid mix of Stable Diffusion + DALL-E) still keeps images looking DALL-Eish (artistic) by default, and I like that.
So, in my opinion, use the generator that meets your need for a specific project.