Solar 2D (formerly Corona SDK) vs B4X

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.

B4XUI custom views are designed, by Erel and other forum members, exactly with this in mind; custom views that act and feel exactly the same no matter what platform you target.

As mentioned in its website, "With B4X, anyone who wants to, can develop real-world solutions."

Erel Uziel is simply the most pro-active Soft-Creator/Forum Manager you will ever come across. If it has not yet been asked & answered, just ask and you will be answered withing minutes, and thanks to it being a trully international forum, this is achieved 24h/7d.

Support for Bluetooth/BLE, MQTT, TCP/UDP, Serial, NFC, Websockets, HTTP/2 across platforms makes it the ideal tool for IoT projects.

A huge productivity boost when you can run your apps on the devices and make code changes real time without needing to recompile.

No need to learn JAVA, C+, PHP, or any other complementary languages in order to achieve exactly what you need.

Produces highly optimised end results. Easy to use and quick to learn. When you know how to produce an Android app you near enough know how to produce an IOS app.

The fact that there are subtle differences between writing for different platforms is a huge plus - those differences help optimise your code and they're really not difficult to wrap your head round.

The speed of answer topics is amazing.

The tutorials help beginners as well as more experienced programmers grasping important concepts.

The Forum is filled with many code examples of fully working apps for free. Code snippets, animation, different type of views etc. can be easily downloaded and used in your own app.

Over the years it has built up a very active user community that is very involved in the evolution of B4X. In their forums you can find any answer to your questions, and if it is not there, they will answer you.

VERY Rapid Application Development, with tons of pro features.

Except the IDE for iOS. Even so it's very cheap.