When comparing Spring-boot vs Symfony, the Slant community recommends Spring-boot for most people. In the question“What are the best backend web frameworks?” Spring-boot is ranked 9th while Symfony is ranked 25th. The most important reason people chose Spring-boot is:
Boot is just a thin configuration layer over Spring Framework, as such it inherits all the strengths of Spring.
Specs
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Pros

Pro Inherits all of Spring's strengths
Boot is just a thin configuration layer over Spring Framework, as such it inherits all the strengths of Spring.

Pro Preconfigured starters

Pro Fast prototyping
Spring boot is built for fast prototyping.

Pro Kick down to Spring
Pro Open Source
Symfony is open source and released under the MIT license.
Pro Easy debugging with a built-in debug toolbar
Symfony comes with a built-in toolbar that helps developers debug their applications during the development phase.
The toolbar is also extendable and new components, called panels can be added if needed to help with the debugging process.
Pro Great plugin ecosystem
One of the greatest strengths of Symfony is it's amazing and large plugin ecosystem, which comes as a result of it's large and dedicated community. Having a large number of plugins means less development time and more productivity.

Pro Highly active community
Symfony has one of the most active communities out of all the PHP frameworks. This is shown by the high number of commits made every day in the GitHub repo.
Pro Teaches you good practices
Symfony makes you be a better programmer. You have to deal with the latest object-oriented design patterns such as service-oriented architecture, dependency injection, interface abstraction, and so on.
Pro Uses YAML/XML/PHP/Annotation
Symfony makes use of XML, YAML or PHP annotations to create configurations in order to tell Doctrine on how properties of a certain class should be.
Pro Powerful event system
Symfony has a powerful built-in event system that allows you to add flexibility to applications and makes it easier to maintain the codebase down the road.
Pro Great templating engine
Uses Twig, which is a simple and easy to learn templating language that can also be used as a standalone engine, outside the framework.
Pro Uses Doctrine ORM
Symfony makes use of the Doctrine ORM to add an abstraction layer over the database in order to maintain flexibility without having unnecessary code duplication.
Cons
Con Lacking in UI development
While actually very good and with a very complete and rich feature set to develop and maintain code on the server side, it still doesn't provide any rich framework for building good user interfaces.
Con Settings
Too many configurations.
Con Very hard to install
Setting it up on webhost without a console is difficult.
Con Promotes bad development practices
Such as annotations via comments.
Con Doctrine ORM
Symfony Standard Edition, which is the most widely used distribution, comes integrated with Doctrine, the most resource hogging ORM library.
Con You need a lot of files to display a single page
For a simple hello world page you need about 5 files.
