When should I use bsp/geometry brushes?!

Hi UE4 Answer hub,
I’m very new to Unreal engine 4, so this is probably a very simple question :slight_smile:
I have just finished drawing my top down sketch of my level and I would like to start blocking it out in UE4.
The level is placed in a mountain environment similar to some of the levels of the new Ratchet and Clank game.
I don’t know if I should use bsp/Geometry brushes or just a terrain?
So far I’ve only seen brushes being used for the interior of buildings, which there is none of in my level.

Would it be efficient to block out an outdoor nature level like mine using brushes?
Or should I just use a terrain?

Any answers will be greatly appreciated!