it is a bit difficult to compare them, but approximately:
- with lower quality stuff I use in MapBuilder (small lod and clipping distances, simple sky cube, simple fog, terrain lod with 2 times smaller vertex resolution, bone animation shader similarly with 400 similar characters, simple shaders especially for lods etc.) and with about max 1000 tree and building entities per level, 3dgs is faster. even if a few thousands of grass models are placed with short clipping (having no collision!).
- but over a certain scene size and detail level, I mean e.g. thousands of trees, 3dgs performance falls rapidly down to unplayable level (I've never used the model merging offered by the TUST library), while UE4 is more stable and optimized for such an environment, even beside an advanced sky system, better shadows, complex vegetation and terrain shaders with physically based rendering, longer view distance of terrain (I try to scale lod distances and clipping approx similarly). probably if I optimize the environment shaders I got from UE4 samples it would perform a bit better. and UE4 renders faster higher poly models thanks to its effective occlusion culling, and other many optimization tricks I don't know.
...and soon I will test it with a nice water, and with many moving and colliding characters on the navmesh (what I will probably replace with my own pathfinder).

basically, working with UE4 goes a bit faster after you get familiar with ue4 editor and its sub-editors:
- for your free subscription you get cool textures, models, materials, animations, and game play codes (or blueprints) you can use freely.
- I find blueprint visual scripting difficult to overview over a certain detail, but cool and fast for simple things.
- the programming side is much more complex! but there are UE4 helper solutions for everything (e.g. dynamic arrays) to make memory handling safer, which requires significant time to get familiar with, especially to build C++ and blueprint classes onto each other smoothly. it is very difficult comparing to lite-c, and requires more preliminary learning and planning!