When comparing UDK vs Shoot 'Em Up Kit, the Slant community recommends UDK for most people. In the question“What are the best 3D game engines?” UDK is ranked 17th while Shoot 'Em Up Kit is ranked 55th. The most important reason people chose UDK is:
UDK has an extremely large toolset that allows creating almost anything without having to use a 3rd party tool or plugin.
Specs
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Pros
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.
Pro NPC Generators
Control NPC spawning position, frequency, maximum enemy active at once, and maximum enemies created by the generator.
Pro Support for user-created HLSL shaders
Both model and post-process shaders.
Pro Great for shoot 'em ups
The name fits the functionality.
Cons
Con Superseded by Unreal Engine 4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.
Con Lacking some details
No information on file formats or codecs in the manual.
Con Tutorials are too short and too old
The only real tutorial videos are 1 or 2 minutes long, and date from 2013.