When comparing Moai SDK vs UDK, the Slant community recommends Moai SDK for most people. In the question“What are the best 2D game engines?” Moai SDK is ranked 19th while UDK is ranked 73rd. The most important reason people chose Moai SDK is:
Because it uses Lua + C++.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Fast
Because it uses Lua + C++.
Pro Gives you total control
You can use your own custom shaders and extend the language with C++. MOAI doesn't give you everything, instead it gives you the tools you need to develop any game you want.
Pro Full source code is available
So it is extendable with C++
Pro Easy-to-use particle engine
Uses a limited subset of Lua
Pro Uses Lua, with LuaJIT where possible

Pro Is used by many pro developers
Moai SDK is used by popular developers including Double Fine for their mobile game, Middle Manager of Justice, and for their Kickstarter hit, Broken Age(formerly, Double Fine Adventure.) It was also used to create the popular mobile game Bubble Ball 2.
This proves that this engine can be used to make fully-featured and fun games.

Pro API interfaces
Has options for development like Hanappe and Rapanui, providing a different way to interact with the engine.
Pro Helpful community
The Moai SDK Forum is active and users will go to great extents to answer queries, post code snippets, beta test, and even purchase and give feedback on each others' games.
Pro Helpful and detailed documentation
Although the Moai SDK is very advanced and uses complicated features, the documentation wiki thoroughly and clearly explains how to use these features, such as the Moai Cloud(a service that provides web services for your game), and multiple ways to build from source.
Pro Large toolset
UDK has an extremely large toolset that allows creating almost anything without having to use a 3rd party tool or plugin.
Pro Triple A track record
UDK is an engine used by many big name companies for popular games such as the Gears of War series.
Cons
Con Not for beginning developers
The Moai SDK has very advanced features which allow it to create powerful games. A drawback from this is that it is not very suitable for beginners.
Con Not necessarily tested or stable

Con Scarce documentation
Since it is a minimalist framework there is not that much documentation, but if you are used to reading API references and source code you are good to go!
Con Development has ceased
The last update was over a year ago. Users should not expect things to still work on future OS versions unless they're ready to fix them yourself.
Con Superseded by Unreal Engine 4
Con DevKit only runs on Windows
Even though UDK can deploy games to run on multiple platforms, including Mac, it does not feature support for development on Mac.
Con Limits games to FPS
UDK was originally developed for the FPS, "Unreal Tournament", and it certainly shows. While UDK is a very powerful engine, it takes quite a bit of work to make it do something other than an FPS.
