GitLab CI vs CircleCI

All build setup are stored in .gitlab-ci.yml file, which is versioned and stored in the project. Like Travis do.

All of GitLab CI's code is open source and under the MIT license.

Tests are parallelized across multiple machines in order to reduce test times considerably.

Good integration with Docker.

The tests of GitLab CI run parallel to each other and are distributed on different machines. Developers can add as many machines as they want or need, making GitLab CI highly scalable to the development team's needs.

Since it uses the GitLab API for setting up hooks, the setup of GitLab CI for projects hosted on GitLab can be done in one click.

Easy to test and deploy on Kubernetes.

CircleCI excels with its setup process. All that's needed is a GitHub login and CircleCI automatically detects the settings for Ruby, Python, Node.js, Java and Clojure. The setup process is their most widely praised feature.

CircleCI can be connected to any project that is hosted on GitHub by logging in using the GitHub OAuth and adding the desired repository.

Whenever a new commit is pushed to GitHub, CircleCI runs the tests that have been already defined and if none of them fails, the build is deployed to the runtime environment.

Users can access the Virtual Machine via SSH and run commands.

In most cases CircleCI automatically get settings from your code. When it fails, edit circle.yml.

Tests can be parallelized across multiple machines reducing test times drastically. They support up to 8-way parallelization. Additionally, CircleCI caches the build environment.

Circle CI's web UI is clean and easy to use.

It gives all the information for a single build in a feed and gives the explanation for each step of the build, what it's doing and what the step is related to. On the top it displays author information and the time and date when the build was started and finished.

This is all done by giving only the most essential information without clogging the screen.

Support for Ruby, Python, Node, Java, PHP, RoR, DJ, JavaScript. It also detects settings for Ruby, Python, Node.js, Java and Clojure.

It als has support for: MySQL, MongoDB, PostgreSQL, Cassandra, Riak, Redis, SQLite, Solr, CouchDB, ElasticSearch, Neo4j, Couchbase, Lucene, Sphinx, ThriftDB, Memcache.

Alongside latest Chrome, Firefox and Webkit (installed using xvfb), CircleCi supports the use of Selenium, PhantomJS as well as tools like Capybara and Cucumber.

Support for RabbitMQ, Beanstalk and Resque through Redis.

CircleCI can continuously deliver Docker images to hosts that support Docker containers.

Eg:
./1.setup.sh 48s
./2.build.sh 56s
With this information, it's easy to find out which line of the script is the bottleneck of the build process.

Can specify the cache dependencies on

1. checksum "package.json"
2. Branch
3. BuildNum
4. Revision
5. Environment.variableName

For more details https://circleci.com/docs/2.0/caching/

CircleCI can notify via email, Hipchat, Campfire and more. And it does so only when necessary.

You can push multiple batches of code concurrently.

Support for Heroku, AWS, Engine Yard, dotCloud, Fabric, Nodejitsu, AppFog, Capistrano, Rockspace, Joynet.

Integration with Heroku is solid with the ability to automatically deploy or merge branches.

CircleCI is also very flexible with the deployment arrangement allowing SSH key management, deployment freedom including directly to a PaaS, using Capistrano, Fabric, arbitrary bash commands, or by auto-merging to another branch, or packaging code up to S3.