When comparing Vaadin vs Ninja, the Slant community recommends Vaadin for most people. In the question“What are the best JVM web frameworks?” Vaadin is ranked 4th while Ninja is ranked 5th. The most important reason people chose Vaadin is:
Ranked in these QuestionsQuestion Ranking
Pro Easy to use
Vaadin has a 'design mode' which allows developers to drag and drop components into a canvas and then provide the logic for every component. There are also a lot of third party tutorials and guides which help with the initial learning curve.
Pro Top notch documentation
The book of Vaadin is a must have for every developer that uses Vaadin to make web apps. It is an excellent reference manual and all around a great tool for every Vaadin related need. You can find and download the pdf online or get it for free in almost any Vaadin sponsored conference.
The online documentation is also very good. It has some tutorials and video guides.
There is also a vibrant community which sorround Vaadin, for any problem you may have there is a big chance that someone has already asked and answered that question on StackOverflow or in the Vaadin forums. If not, it will probably be answered quickly if you ask it.
Pro Modern, configurable theming
Vaadin comes with the Valo Theme, a Sass-based theme and engine that calculates styles based on configurable variables and making it easy to define a completely new theme with a few lines of SCSS.
Pro Easy to develop prototypes quickly
Vaadin has what is called the Vaadin Directory which is a repository of downloadable Vaadin components which can be easily used for development. To use something from the repository, you need to download the JAR file and add it to the project.
Pro Simple set up
Once dependencies like maven are installed it is up and running in minutes with one simple command.
Pro Easy horizontal scaling
Ninja is stateless by design. This makes horizontal scaling very easy and just a matter of adding servers.
Con Weak scalability
Since Vaadin stores the UI state and logic in the server, this means that for every user interaction a request needs to be sent to the server and the client needs to wait so it can know how to react. This leads to higher traffic and load times.
Con Little user choice in organization
Since most of the code and folder structure are automatically generated, this leaves little room to the developer on how they will organize their project.