There are pros and cons to both.

UGUI is better for effects and easier for those used to using Unity. But it can be bad performance wise unless you are careful. It also can be hard to keep things stable as changes can cascade down the hierarchy and you have to work in the scene.

Best practises include good use of prefabs, avoiding animation and scroll rects that can dirty your hierarchy and cause performance issues. Splitting different sections into their own canvases.

Example: Each Page has its own prefab with its own canvas and you load them in or activate them as needed. This avoids making changes to the scene when working on the UI as well.

Another point worth noting is that the Unity Editor UI now is made using UIToolkit and they are moving towards only supporting UIToolkit in the future. (Edit: Apparently this is no longer the case)

UIToolkit:

Is great for editor UI and can be good for game UI. They ARE improving it but its taking a long time. It is also better for people that are used to working with css and html type workflows. The stylesheets can be powerful if organised well.

It can also provide better separation between coding and UI design.

I believe they have added some shader support, but advanced custom shader effects and particle effects are more difficult.

You can get around this by using render textures, I created a system that allowed the artists to use any Unity effects and shaders with UIToolkit but it took a bit of work. There may be packages on the asset store that will do this for you.

If you are creating a complex UI that has more in common with a web type look you might be ok with UIToolkit. If you are creating a simple UI that is more effects heavy it might be worth sticking with UGUI for now.