Solar 2D (formerly Corona SDK) vs Skyline

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.

A growing community that is always helpful with dev response times averaging less then 24 hours. Often less than 12.

Skyline provides a straightforward drag and drop development environment that can be customized to suit individual preferences. Playing the game and animating the scene can be done from within the editor at any time.

Future versions will allow extending editor's functionality with plugins and offer an official way of trading custom plugins.

Skyline's commercial edition costs just £40, has no royalties or earning limitations and includes 2 activations per license purchase and a year of free updates.

Skyline can be run on a machine with 2GHz Dual Core CPU, 2GB of RAM and Nvidia's 9800 GT (or equivalent) GPU. While, obviously, higher specs are recommended, it's reasonable to expect around 60fps on small scenes with these specs.

The Terrain editor is easy to use and will include features such as hole cutting.
There is a differed lighting render for internal scenes, which makes creating multi light and beautiful/atmospheric scenes so much easier.

All the tools for the terrain are in one place such as placing road/paths, vegetation, billboards, and the settings are easy to locate and change.

Planned features include:
Hole cutting in terrain and
Terrain paging.

Making a game in the current top engines is hard work, making something small and basic in Skyline is not. Easy to learn drag an drop model placement and editing supplemented with easy to follow video's on forum make the process straightforward.

Including image based lighting.

A free version that's limited to non-commercial usage and lacks a few features is available.

Rendering of a water plane looks fantastic and is easily edited for different effects.

The schematic editor is designed to make it easier for non-programmers to create games.

Not much in it at the moment but is starting to grow and will continue to grow with an increase of users.

There are many additions to the engine that are on the road map such as multi threading and tree physics.
The feature list is constantly evolving with each user request and idea put forth.

Skyline has a custom editor that gives the user of making their own tools for use directly in the editor to develop their games. All the editors are made using the Qt Designer and programmed in lua inside the engine itself.

Open-Source Virtual Reality support is in development.

Shadow editor.
Weapon Class Editor.
Mesh Editor.
Advanced Mesh Editor.
GUI Editor.
Script Editor.
Environment Editor (volumetric clouds and day night cycle, lightening, ocean settings, height fog).
Particle Editor.
Path editor.
Schematic editor.
Navmesh editor.
Material editor.

The documentation is been rewritten and follows a structure like a course.
Introduction to > Level designer with examples > Artists

There is also video tutorials, video examples and video tutorial series been produced.

There are user created demo scenes, examples and more.

Making HUDs and GUIs has never been easier

Full PBR workflow, your not stuck to Secular or Metallic it has both

Can contact the Skyline developers and they will always respond and help as fast as they can.