Solar 2D (formerly Corona SDK) vs GameGuru

Corona SDK is completely free. That includes pro-tier plugins.

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.

It's easy to create a game that looks good on many different sized mobile devices.

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.

Uses the great and easy-to-learn Lua programming language.

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.

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.

Corona works on OS X, Windows and Android (including Kindle Fire & Nook).

Since Corona SDK became Solar2D, it's completely free, as only some third-party plugins are paid.

Since Corona SDK became Solar2D, it's completely open source under MIT license.

Simplicity and accessibility are the core design goals of FPS Creator Reloaded.

Users share ideas, help, and advice through the official forums. The community also provides a wealth of free assets, and the GameGuru Assets Store sees constant additions.

News and improvements are released on a monthly schedule. Interaction between The Game Creators and project backers is excellent.

Easily import your own 3D models with the built-in importer and create custom characters with the Character creator. Custom weapons are much more difficult, though.

The gamer creator forums have a large number of users, developing games, making models for sale and to give away for other users. They answer questions, help each other with problems in their games, and can make using this software so much more fun and easy.

Games you publish with GameGuru can be sold without any restrictions or commissions.

An easy 3D editor to create your own maps in seconds with drag-and-drop function.

Awesome friendly GUI, and quite easy to use.

Currently in development but already looking very good! Easily create big and customized buildings with default building blocks and share them with others.

Also very easy to use in the designer: The buildings are treated like entities but still directly available in the 'building editor mode'.

Uses Lua.

A store where you can buy various assets for your games like sounds, 3D models, weapons... Very good prices and a lot of assets are even free.

The Game Creators team frequent the forums and ask the community for feeback and suggestions both there and via an open poll on the website. They have a great history of reading suggestions, viewing screenshots, and generally working with the community.

The game engine is optimized for efficient use of memory.