Quote: "I cannot choose between the 2 game engines, Unity and Unreal"
And you ask this here? - OK, I'll read on...
Quote: "they both seem very good for beginners"
Really? Which part did you find "very good" for beginners?. I'm guessing the drag and drop parts but I'll read on.
Quote: "My "game" will mainly be artwork. Lots of foliage. Trees, plants, and bushes"
That's not a game, that's just creating a scene.
Quote: "I am not very good at code right at the moment. The main thing I need my game to have is excellent graphics, and performance"
I want that too. Bring on the excellent graphics and performance.. Where are you getting yours from?
Quote: "I will try to make as realistic as possible. For a lot of the game development will probably be drag and drop"
Again, what you are describing is scene building and just one small part of creating a game. Also dragging and dropping won't make it realistic. You don't think you can just drag and drop a tree model and have it swaying in the wind with the sun shining though it's branches and leaves did you?.
Sorry to be harsh but your expectations are way short of reality. Dreams are great and I'd say go for them but don't ask how to make those dreams a reality until you've at least gained some experience in reality first.