Why decide? Why not create portfolio pieces in both engines?
Thing is, Unity and Unreal are two of the most used engines among hobbyists and Indies, and are also quite wellknown and used among bigger studios. Both have their Pros and Cons, but that shouldn't really bother you. You want to show potential employers that you can make ANY engine shine, and not that you are good at picking the engine that makes your work shine.
If you find work in a bigger studio, chances are higher than 80% that you will not work in either of those engines. There are a ton of other engines being employed by bigger studios, some not as hobbyist / Indie / Beginner friendly because of price or features... others not really available to people outside the studio, or at least without working for a certain publisher. Frostbite comes to mind.
Now, getting back to the wellknown part, people in the industry know both those engines even if they do not work with them. It doesn't matter much if your UE4 showcase looks better than the Unity one (and let me tell you, if you are good, and have the right tools or enough time, both will look just as good)... as long as it looks good for the engine it runs in, people will react positive to it.
Don't forget the people that will look at these portfolio pieces are not consumers/gamers... they are not looking for the most shiny effects... thos effects actually can get in the way of seeing the work you have done (e.g. strong postprocess effects).
They want to see what YOU did. And that has nothing to do with the glitz the engine adds to your work. They want to see passion, hard work and skill. No engine no matter how advanced can make up for missing passion, and a simpler tool can make it actually easier for you to show hard work and skill.
You want to show your potential employer that you have the basic skills to use modern engines, and that you are able to adapt to ANY engine thrown your way. Because you WILL have to adapt, the studio will not adapt to your wishes. So showing work done in multiple engines will most probably be a very good idea.
Now, getting 3 to 4 steps back, I have to ask why the question of which engine to use is even important if you are trying to apply as a 3D Artist. Your tool is Maya, or 3DS Max, or Blender, or ZBrush (or all of them together)... it is good to show you are also able to find your way around in a game engine, so you are able to import and test your models yourself and the level designers can be sure to get well tested and working models from you, without having to send them back 3 times for fixing bugs you would have spotted immidiatly if you would have tested them in-engine.
But really, your job as a 3D Modeller is NOT to create levels, or game logic, or hook it all up. If that is what you are interested in, you might want to go for level designer.
If you are actually more interested in everything together, then working in a bigger studio might not be ideal for you. If you can go with a most probably slimmer paycheck, such jack-of-all-trades might be in higher demand in small Indie shops and studios...
Don't get me wrong, having an animated or even ingame showcase for you models certainly will make your work look even better, and show additional skillsets above and beyond what is expected of a 3D artist... but really, this is all nice to have. Before layering all the glitz on your showcase by "picking a better engine", "that has better postprocess effects", "has better performance" and whatnot, make sure the foundation is in place. Good models, no import problems, a very good showcase showing off your models, and enough variety to make sure your future employer has one example that might match what is expected of their future artist best.
In the end, its the quality of your 3D Models that counts.