AFAIK Unity and UE4 are pretty much on par when it comes to usability and features.
The ease of use of editors and tools are most probably more or less on par, altough I never took the time to really compare it (others will be able to help more here). I can only say I am a long time Unity users and apart of some small things, it is a fantastic toolset that lets you do almost anything with ease. Of course, IF YOU ARE EXPIERIENCED ENOUGH!
Now, from what I have seen, UE4 might be a little bit more powerful with Unity only catching up with version 5 coming out in the next few months, and UE4 still having some highend features already added Unity currently is lacking (like temporal AA for example)... Unity 5 will have its own set of features not in UE4 natively yet (like enlighten), but anyway, featurewise they will be more or leass on par if you include available 3rd party tools.
I would say, performancewise UE4 might have a slight edge against Unity 4 because parts of its Engine core is still pretty dated and is using patched together solutions (for example no REAL deferred renderer, just deferred lighting)... that might change with Unity 5 though (a true deferred renderer is promised).
On the other hand, I would give Unity a slight advantage when it comes to documentation and community. I didn't really had a big look into the UDK and UE4 documentation and community, but Unity has longer been in the Indie-Scene-Game, their documentation is outstanding, the community is vast, and there are literally tousands of third party developers creating plugins, shaders and tools for it and distributing it over unitys asset store (part of it is even free).
The real difference comes in when you look at pricing. There is no better solution here, just different price points for different people:
1) cheapest is Unity Free. It is, as the name says, free of charge. You get some cut features like post processing, render textures, no deferred renderer, less batching options, no profiler. This will ONLY really matter when you are targetting really high-end graphics though.
2) UE4 comes close to Unity Free: it can be yours for 20$. that is right, you pay 20$ for a single months subscription, download the engine, and cancel your subscription. You can continue to use the engine, you will just have to pay 20$ again to get updates.
A warning though: If the product you create with UE4 makes of 50k$, you will have to pay 5% royalities for everything over that 50k$.
3) Unity Pro gives you the full Unity toolset: 1500$ per seat (or the equivalent in monthly or yearly subs... you will not save money that way, as you pay the same after a little more than a year). Yes, it is expensive. If you develop something that makes some money later on though, you will save that amount of money with ease though. 1500$ equals what you pay in royalities for UE4 if you make >80k$ with your app or game.
Now as a fair word of warning: to you as a newcomer, the difference between these options will be small. You will have a hard time really using all the options coming with Unity Free as a newcomer lone wolf, save all the advanced options you get with Unity Pro or UE4.
If you are ready to sink 20$ into getting UE4 you will certainly get a more powerful toolset than you would get with Unity free, just be aware, you might pay that 3-4 times a year, if an important update for UE comes out you cannot or don't want to skip, so factor in a yearly cost of 50-100$ over the course of your work with UE4.
But you need to be aware of the vast amount of time and skill needed for even a simple 3D game, you will need to be able to program, you will need 3D models (meaning you either learn the skills for 3D modelling or shelling out bucks for stock art or freelancing, IF you don't find free models that do the job), and you will need to learn a lot of basics about 3D rendering and engines, before you can even start to work on your projects.
Really, stick with Unity Free until you get a fair understanding of 3D Engines and how to create games. Moving over from Unity to UE4 is possible at any time, and while C++ is not C#, and the Unity Editor is different from the Unreal Editor, 60-75% of your knowledge will still be transferrable. Don't worry too much about the tools you use, worry about honing your general and basic skills that will apply to all tools and languages you will use over the course of your career (professional or hobby doesn't matter), and worry about your efficiency and planing skills (Most probably you have no idea yet how many years of your life the game you dream about at the moment might consume, if you don't compromise a lot and get much better skills)
Unity is much more mature and the assest store has a good selection, unreal is still starting off.
Uh... Unreal has been around since 1996, Unity wasn't even announced until 2005
Unreal is pretty new to the indie scene nonetheless. Until UDK, only big studios were able to afford Unreal, and even UDK was, while much more powerful than Unity 3.X, held back by that attrociously high royalities of 25%.
Unity has been targetting Indies and hobbiests from the start. It always had a free version, and an asset store. That is why Unity still is in much more demand among the smaller Indie studios than Unreal.
Depending on how much traction Unreal gets with its radically changed pricing structure and full source for UE4, that might change. On the other hand, for the first time in Unitys history its engine is no longer far behind the big commercial ones when it comes to features and performance with Unity 5, so who knows, maybe Unity will revamp its own pricing again (as it has in the past as a reaction to UE4 pricing changes... thanks to that, you can now target mobile for free, you get hard shadows in the free version (which can be turned into soft shadows with an inexpensive asset store plugin)).
I wouldn't call Unity more mature than Unreal, Unity until Unity 4 had an very dated engine core, and even in Unity 4 there is some legacy stuff that needs work. Unity 5 might see Unity finally catching up.
Still, the Asset Store is a fair point, Unity's asset store is most probably the best stocked asset store out there.
And really, Unreal IS kinda new to the low cost engine game.... At least Epic recognized both the danger of Unity becomming a threat (which kinda forced its way into lots of smaller studios and also schools lately), and the money that could be gained by hooking startups and new devs to its tools, so they continue to use it when they finally have the money to pay the full license...