I am familiar with both. A Unity game I developed a ton of modular characters for is about to be published, so I'd hope that would be enough to show I understand how models work in a game engine. It's just a matter of taking a week to do some studying and messing around in UE4 to get the same quality I'd get in Toolbag.…
Short answer: For character artists, and prop artists, you don't need to use anything else than Marmoset Toolbag 3... but for environment artists, it's important to know a fully featured game engine such as Unity or UE4, because of tiling materials, modular meshes with precise snapping (pivots) and lighting + lightmap…