Both. You'll probably use vert paint, being cheap and effective way to blend materials, and probably masks, even if it's just a basic noise and gradient material function to break up vertex paint blending or height lerp from a height imput for example. Also don't forget decals on top of what ever solution you use.
Hey there^^ I'm interested in your project. please contact me. Here is my Discord I'd : strom_01 I will show you some examples for you on there. I hope you like my art style Please let me know