You need to profile and see, but in general you'd want to use dedicated terrain rendering techniques for rendering terrain. Not a bunch of static meshes imported from model files.
There are optimizations that can be made for terrain rendering that take advantage of the constraints of the terrain: it is height-map based, and it's on a regular grid, et cetera. Terrain rendering typically uses a subsection of this overall grid as the unit of optimization for falling back to lower-LOD versions, and the geometry for the terrain can be produced on-demand on the GPU as a consequence of the same.
Static meshes are themselves a unit of optimization, and usually aren't as easy to LOD efficiently and automatically in a shader (in the general case). So while in theory you could implement some of these optimizations yourself, you'd be implementing them yourself. They come "for free" with systems that are dedicated to terrain rendering.
The cost of a mesh is fixed, and generally large for something high-resolution enough to be a reasonable-sized terrain chunk. In contrast, the cost of the mesh data for terrain is effectively 0 (it can be generated, since it's based on a height map). The texture cost for the heightmap isn't too onerous; even though it's probably larger dimension-wise, it's probably smaller channel-wise. So it's very possible that the dedicated terrain approach will both load faster and use less memory, depending on how you subdivide your world, your resolution choices, et cetera.
You will find similar tradeoffs in the authoring tools. Unreal's terrain tools allow you to do complex tasks like placing spline-based features over the terrain that are snapped to the terrain (or deform the terrain around it). These splines can have tailored optimizations to processing and rendering just like the terrain can. Static meshes don't, and so you may find yourself with a tedious content-re-authoring pipeline to make any kind of changes to these kinds of features because you need to "bake" them into the meshes themselves.
Every game is different, and only you can do the analysis on your game to determine how the pros and cons balance out. But if I were to bet, I'd bet you'll have better results by using Unreal's terrain tools, both in terms of performance and in terms of ergonomics in the editor.
