When comparing Duality vs Adventure Game Studio, the Slant community recommends Adventure Game Studio for most people. In the question“What are the best 2D game engines?” Adventure Game Studio is ranked 23rd while Duality is ranked 30th. The most important reason people chose Adventure Game Studio is:
Good for newbie game creators. Can be used for prototyping: on several occasions was used to make a demo/experimental version before creating a final commercial product on different engine.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Open source
The full source code is available on GitHub, where the framework is actively developed. All of the editor, core and plugin code is written in C#.

Pro Great editor
Duality includes a powerful visual editing system that can be used for previewing, integrating, editing and testing game content.

Pro Live reload of code and assets

Pro Very extensible
Because both Core and Editor are completely plugin-based, Duality not only incorporates a clean and modular design, but is also very extensible - even if it wasn't Open Source. In fact, most of the standard editing capabilities comes in form of Editor plugins.

Pro Built-in physics and lighting

Pro Friendly to version control systems
It can be configured to serialize all data in a text-based format, which has been structurally optimized for version control systems.

Pro Used in commercial projects
It has been used in a production environment without burning the place to the ground. Supposedly artist-proof editor workflow with an API for tailoring the system to fit your team.
Pro Easy to learn tool
Good for newbie game creators. Can be used for prototyping: on several occasions was used to make a demo/experimental version before creating a final commercial product on different engine.
Pro Completely free and open source
AGS is licensed under Artistic License 2.0 and is completely free for use for creating both freeware and commercial games.
Pro Relatively well documented
Besides the manual there are multiple text and video tutorials and code samples written by community.
Pro Used for a number of high-profile commercial releases
Adventure Game Studio has been used to develop games such as "Resonance", "Blackwell" series, "Gemini Rue", "Primordia".
Pro Lots of assets available
An extensive library of game templates and script modules accumulated over years. You can construct a simple game in hours (if you know what you are doing).
Pro Friendly community
An old, big and active community which would support newcomers not only in learning basics of the engine, but can help with every aspect of game making (including art, voice acting, moral support, etc).
Cons
Con Visual Studio required
To have possibility for scripting you have to download entire Visual Studio and spend 10 GB of free space
Con Requires windows for development
Con Natively supports only 2D
2D only native support, 3D could be supported with plugins though.
Con Uses dated tech
Engine is based on the old technologies, which impose number of limitations and may cause problems on latest systems (level of annoyance varies depending on your priorities).
Con Graphics renderer is a bit dated
Graphics renderer is not well optimized for high-resolution games and complex effects.
Con No visual editor for scripts
You have to actually write all scripts yourself.
Con Development is slow
Further development of the engine is currently slow, done by only few people in their free time.
Con AGS Script isn't as full-featured as other scripting languages
Its own scripting language has lower syntax capabilities compared to modern script languages.
Con Assets cover almost exclusively adventure/quest genre
The features, script functions and game templates are very biased towards adventure/quest genre. The non-adventure games were made in AGS (2D shooters, platformers, turn-based strategies), but their development usually requires to write everything from scratch.
Con Workflow is closely coupled with the editor
Workflow is very tied to the editor and custom file formats, which can cause problems for bigger, more professional projects (interfering with source control, parallel development, automated builds, etc)
