When comparing Solar 2D (formerly Corona SDK) vs Unity, the Slant community recommends Solar 2D (formerly Corona SDK) for most people. In the question“What are the best 100% free and easy game engines for beginners?” Solar 2D (formerly Corona SDK) is ranked 5th while Unity is ranked 22nd.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Very simple to use

Pro Free
Corona SDK is completely free. That includes pro-tier plugins.
Pro Corona Simulator
Corona SDK ships with Corona Simulator, which runs your game/app directly on your PC/Mac and updates every time you make changes.
It provides immediate feedback to your actions, you can see your changes right on the screen, without necessity to make build to device. Getting instant feedback really boosts tenfold prototyping and development speed.

Pro Good documentation and lots of tutorials
Pro Content scaling
It's easy to create a game that looks good on many different sized mobile devices.
Pro Live builds - update builds running on a device automatically
With the live build feature, once you have created a build and installed on a device, you get lightning fast turnaround times because any change on the code or data is updated to the devices running the game (within the local WiFi) immediately. So changes can be tested on the real hardware within a very few seconds.
What's even more impressive, this even works flawless with multiple devices running the game. You have to use it to learn how good of a feature this is while development and even more, while doing QA. Imagine fixing bugs and everyone of your QA team/friends/whoever helps to get your game done, has all changes on his device without doing anything but waiting 5 seconds - outstanding.
Pro Lua syntax
Uses the great and easy-to-learn Lua programming language.
Pro Very comprehensive API
It's very quick to get things up and running with Corona SDK. The API is extensive and while it's not 100% feature-complete with the iOS API, it's close enough that you could create tons of games and never run into a roadblock.
The API docs can be found here.
Pro Amazing learning curve
Corona does not throw photoshop-like madness full of buttons editor. You can go as fast as you want, learning and building game from ground up. Eventually, you'll learn how much corona is doing for you. But to start you don't have to master complex editor software. It's a great tool to learn to start game development if you want to learn how to program and make games. Your experience will be 100% transferable to any other Pro game engine.

Pro Cross-platform desktop and mobile
Corona works on OS X, Windows and Android (including Kindle Fire & Nook).
Pro Marketplace for 3rd party plug-ins

Pro Great community

Pro Ability to call any native (C/C++/Obj-C/Java) library
Pro Completely free
Since Corona SDK became Solar2D, it's completely free, as only some third-party plugins are paid.
Pro Open Source
Since Corona SDK became Solar2D, it's completely open source under MIT license.

Pro Well supported

Pro Great community support
Unity is bolstered by a huge, helpful community via its official forums, wiki, Unity Answers, and Unity Asset Store; plus the most-subscribed subreddit of any game engine.
Pro OUYA support
Pro Over 20 platforms
Unity offers over 20 platforms for publishing including mobile, console, web, VR, and more.
Pro Versatile
Not tailored for specific types of games (like Unreal...), so it won't get in your way if you want to make something unique.
Pro Well structured
Overall, a coherent engine with a rational approach. People who complain a lot about being forced to hack around it usually do not read the docs, like the one that describes orders of execution, or specific functions hooks and such. Some like to say it lacks raw power where people who are used to standard optimizations have no problem. For example It is not uncommon to encounter users who complain about low FPS but forgot to activate occlusion, flag static elements, activate animations culling, and so on. As for complaints about C#, people who are transitioning from C++ were already bad at C++ before being bad at C#. They often come from the PC world where the sheer power of today's machines is very forgiving compared to the platforms we had to develop for in the 80s~90s. One of their errors is for example to never read this doc.
Pro Very optimized
Unity runs very smoothly even on systems that are considered "weak" by today's standards.
Pro Lots of assets can be found in the Asset Store
For those developers who can't afford an artist, or aren't skilled enough to create their own art, Unity features an Asset Store full of a wide variety of free and paid assets that can be easily added to a game. The Asset Store has more than just music and art. It also has code and modules that can be added to games including unique lighting or GUI systems. It also has powerful asset management and attribute inspection.
Pro Works with 3rd party IDEs
You can use any C# IDE for it, but the ones tested which have Unity integration are:
- Microsoft Visual Studio
- MonoDevelop
- Visual Studio Code (much faster than VS, but a bit harder to set up for Unity development)
- JetBrains Rider (very fast, has lots of functionality and best Unity integration, but it is not free)
Pro Flexibility is provided by a strong component programming model
Pro Has a great animation system
Unity provides a great state machine animation system called Mechanim allowing to separate animation from the model and assign the same animoations to different models.
Pro Powerful standard shaders
The built in standard shader in Unity 5 is incredibly optimized and supports PBS/PBR.
Pro Allows for rapid prototyping
Unity's modular system and usability allows for quickly developing a prototype of an idea. It has features like drag & drop editing, shaders, animation and other systems already in place to allow diving right into developing a game.
Pro Easy learning curve
The way the editor is structured, by setting scripts on objects, and the use of a high-level language, C#, makes it easy to learn.
Pro Can create custom forms and tools
Pro Very popular
Unity is a proven game engine. It is used by a wide range of developers - from small indies to triple-A companies such as Microsoft, Paradox, Square Enix and Sega.
Pro Lots of resources to learn from
Unity3D provides an exhaustive documentation where everything is given a full description supplied by a number of examples as well as video and text tutorials and live training sessions to understand the ins and outs of the engine. In addition there's an ever-growing community that can offer advice to help resolve any situations that may arise.
Along with the official Unity resources, there are many high quality (and often free) third party tutorials available.
Pro Great editor
The editor GUI is very powerful and intuitive. It allows pausing gameplay and manipulating the scene at any time as well as progress gameplay frame by frame. It also has powerful asset management and attribute inspection.
This allows it to be more powerful than other, simpler drag-and-drop engines such as Game Maker Studio, although it can take a bit more experience to learn the workflow.
Pro As of Unity 5 all engine features are free for everyone
As long as the company makes $100k or less, Unity's free version can be used to release games without purchasing the pro version.
Pro Supports 2D and 3D
With Unity knowledge of just one engine is needed to be able to create both 2D and 3D games.
Pro Great community
Great Community support through Forums and Unity Answers.
Pro Provides access to a huge list of assets through Asset Store
There's an Asset Store, providing free and paid assets (including components). It also has powerful asset management and attribute inspection.
Cons
Con Free, but not completely
Con Making a device build requires internet connection
To build your app for the device (iOS/Android/AppleTV) Corona requires to fetch resources from online. This would include base application template and plugins. This allows not to perform local build or use Xcode or Android Studio to do a build. Even Large games/apps would build very fast with good internet connection.
Your code never leaves computed. Corona SDK would transfer some information to determine which plugins and pieces has to be transferred in order to make a final steps in build.
As a bonus - you get basically one button press to get from your Corona Simulator game to game on a device.

Con No native editor for Linux
Though it builds for Linux, the Unity Editor runs only on Windows and Mac. Some users have successfully run the Windows version on Linux via Wine.
