Noob question about map file and importing

Noob question here

I’ve been using unreal engine for almost a year now building up a dream game of mine however i haven’t attempted to build the world/map yet (Mostly focusing on the character movement and abilities) my question is would it be better to start a new project just to work on the world map and somehow import to my game project or is it better to do everything in the one project?

I would love to get feedback on this subject and to why you would go whatever direction you suggest thanks in advanced.