it really depends on what you need, what are your preferences. the best to test both

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with simple graphics and simple scenes 3dgs performs well, easy to use, flexible, basically I like it. you can start with very basic knowledge, and go forward, but if you want to make some high quality stuff, you have to make your own tools, development environment (I made MapBuilder because WED is from the nineties), and shader package, performance falls very rapidly with complexity (entity and view quantity), difficult to get high quality graphics assets for free/cheap price, difficult to establish a team. I need large open scenes, where 3dgs even with the Pro edition has certain limits I would like to break through.

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with UE4 you get a huge complex high quality package, requiring much longer time to get familiar with, optimized for modern hardware, basically for the future, which is great as game development is not an instant thing. so your potential players are different (I would not say limited). it means you have a lower basic FPS, but it falls not so much as you increase quality or complexity, thanks to several optimizations. and in UE4 editor there are simple scalability options to decrease quality and get more power, I use it intensively, because my notebook is just over minimal requirements, but apparently I can approximate 3DGS performance beside a slightly higher quality and scene complexity (it is not possible to set the same test environment, the renderer is very different). I will see how it goes with a real RTS system after I port my pathfinding solution (the built in navmesh is fast but rather for 1st/3rd person games with relatively similar enemies, and with limited AI decision making possibilities).

when you start a new project with UE4 you can check the available templates both for Blueprints and C++, thus you can get a basic character with animation and movement stuff, and it is not so bad to set up simple enemy AI behaviour, which is a long work with 3dgs. and there are several free to use samples and sample games with free assets, and game-play related solutions, directly from professional developers.

moreover, there is a backward support for Win XP made recently, targeting lower end hardware by using OpenGL instead of DirectX, but I have not tested yet...