When comparing Allegro vs Solar 2D (formerly Corona SDK), the Slant community recommends Allegro for most people. In the question“What are the best 2D game engines?” Allegro is ranked 12th while Solar 2D (formerly Corona SDK) is ranked 15th. The most important reason people chose Allegro is:
The Allegro community has produced a lot of great tutorials and resources. Allegro [Wiki](https://wiki.allegro.cc/index.php?title=Main_Page), Mike Geig's Allegro [Tutorials](http://fixbyproximity.com/2d-game-development-course/), Rachel Morris' [Tutorials](https://www.youtube.com/watch?v=E4RqHtEAAds), CodingMadeEasy's [Tutorials](https://www.youtube.com/playlist?list=PL6B459AAE1642C8B4&feature=plcp).
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Supports desktop and mobile
Support for Windows, Mac OS X, Linux, iPhone, and Android

Pro Good engine architecture
Allegro is well designed, easy to use and has many useful features.
Pro Good documentation and lots of tutorials
Since it has been in development since mid-90s with hundreds of people contributing to both the engine and documentation, it has all of its bases covered when it comes to standard support.

Pro Per-platform library optimization
Allegro uses DirectX for Windows, and OpenGL for other targets.
Pro Freedom to implement your own game engine
You are not bound to the limits of existing game engines, and you can actually implement your own engine.
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
Cons
Con Learning curve for hobbyist developers
Hobbyist developers coding alone may experience a learning curve with Allegro of about 200 hours (if you are rusty on C++). To learn quickly, see Mike Geig's tutorials at Fix By Proximity. This learning curve may be fine if you are considering going professional, but are still unsure.
For hobbyist developers not planning on going professional, you may want to look into a complete 2D game engine, rather than a coding library. For example, there are "non-coding" engines that provide support for coded plugins or scripting. But, if you are a dedicated hobbyist planning to use Allegro as your coding library of choice, you can still develop great games as a hobbyist.
Con Isn't great for C++
If you are a fan of object oriented programming, and want to use this library, then the chances are that you are going to be creating a lot of wrappers for functions in this library.
In short, if you're a C++ person, it could be recommended to check out SFML instead.
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.
