Isn't the main problem with garbage-collected languages unpredictable and possibly noticeable GC pauses? Obviously languages with a runtime get other performance penalty as well, but I'm still under the impression that the biggest culprits are bad software design and GC pauses.

Unreal Engine 4 is technically superior, has better documentation and support, tools and generally has a far superior development pipeline.

Yooka Laylee is probably the greatest exception. I mean that's kind of presentation you would expect from a UE4 game and post patches has improved.

An honorable mention would be Recore. I know the original X1 release was a mess but it was always a pretty decent looking PC game.

Unity is proving to be a capable engine at best, but UE4 is still one of the most impressive.

Union Europeene IV...

...The hell happened to CryEngine5?

The end product is rarely an indication of the engine itself but its clientele. More amateurs use Unity and more professionals use UE. I'm pretty sure you can get the same end result with both. In the end they're just tools, which in qualified hands, will result in a similar end product.

So you won't get a proper answer from anyone who hasn't worked professionally with both. Hell, in this forum most people don't even know what an engine actually is and does.

Unity uses C# and UE4 uses C++.
Unity only costs a one-time fee to publish games, UE4 is free to use but costs royalties from whatever profit you gain from selling the game (including profit from Ads).
I know that UE4 would be the best option atm for certain AAA Console/PC games with the more lighting/shader options but Unity is growing in popularity and (With continuous upgrades) it may overtake UE4 in years to come.
If your just making an indie or mobile game, choosing either software is understandable.

As for me, I just make 2D games in Gamemaker atm, but I'm a 3D animator who uses Maya and software alike and I've been thinking of making a 3D game for a while, I've already built a good asset in Maya (character modeled + textured. but not rigged or animated yet) with a clear game design in my head and I've been wondering which 3D game-dev software to use too. I was actually going to use Unity (to create a character controller then go on from there) as unity is really popular in the industry, only a one-time fee for a publishing license (unless for a console publishing license which has a 12 month expiry date), there are tones of tutorials and it's popularity is increasing. But I may look into UE4 for another project (Even though I'm not a AAA developer) as it is free until I make a profit plus the lighting/shaders are really nice but I hope there is as many tutorials for UE4 as there is with Unity. I'm sure they'll be plenty of tutorials out there for both :)

Yeah Yooka Laylee and Recore are probably the best examples I've seen. Still though, I'd say UE4 titles like Gears of War 4 and Hellblade eclipse those comfortably.

Yeah, GC pauses result in terrible performance until they finish. Most GC implementations require all threads to sleep while GC is taking place (they can't handle simultaneous changes to the graph of program memory while walking said graph to find memory islands). The bad software design angle the result of bad programming/bad management. What you have to watch out for is twisting your program's design to avoid the garbage collector (preallocating arrays/objects, etc.). C# just isn't a good language for high performance (AAA/AA) games because of said garbage collector. Indie games may get away with it, but you really have to know unity to make a good, performant game in it (many don't, which is why poor performing Unity games get released all the time). That said, there is something to heap compaction. On the other hand, needing to use new (which allocates memory) everywhere is definitely a bad thing. 

Handle tables are pretty cool though. You can do that in C/C++ just fine. C++ is another bag of worms all together. Not a fan of C++ to say the least. If I had a say, I'd just use plain C. Easier to parse (meta-programming), easier to use, and easier to estimate the performance of resulting assembly code.

I'm pretty sure Unity is totally free until you make $100,000 a year.