Depends on the model and what you are generating. Stable Diffusion is legit insane for the number and variety of models you can use. But it's very much the "custom mods" version of these image generators that can really isolate down the very specific things you want to create and let you train your own model. Something like MidJourney however is more of a general, very broad image/art creator that sort of has its own style and aesthetic, which makes it better for a lot of detail and complexity in any sort of general thing you might want to create on the fly.

In other words:

• A more serious and detail envisioned project will be better with the ocean of tools and custom controls on Stable Diffusion.

• A simpler, casual image generation to get some neat looking stuff with little vision of what you are looking for will get a better result with MidJourney.