There is a clear winner if you ask me, and the answer might surprise you. Disclaimer: It is not a popular opinion. Without much thought, everybody is just straight adding RSpec to their codebase by default. I believe that is not the right thing to do, and here are my reasons why.

What is minitest?

Minitest is a minimalistic testing framework that is fast and easy to learn. It doesn't have as many features as RSpec, but Minitest is more than adequate for any project. For me, it is a clear winner. Usually, I do not care much what most people think or do if I do not believe it is correct.

Why is minitest better?

These are the main reasons why I prefer minitest over RSpec:

1. Speed

In general, Minitest is faster than RSpec when running tests. This means that you can get feedback about your code more quickly. Making it easier to find and fix errors and improve the quality of your code.

There are many reasons why minitest is faster than RSpec. First and foremost, minitest is a much smaller and simpler gem than RSpec. This means that it takes up less memory and is quicker to load. Simply put, it just has a much smaller codebase than RSpec.

All in all, minitest is no-questions faster. Speed and the ability to re-run the whole test suite locally are of utmost importance for me.

2. Ease of Use

Minitest is more concise, which makes it a more efficient tool. Additionally, it has a more intuitive syntax, making the learning process more convenient. It is simpler to use than RSpec, making it a perfect candidate for beginners.

Unfortunately, on the other hand, RSpec is more feature-rich than minitest. It is more popular with developers who favor a more 'spec-based' testing approach(having the ability to deep nest describe/context blocks). Minitest has no this feature by default(you would have to use the minitest/spec library). But this feature comes with a price: RSpec is definitely more difficult to learn than minitest.

3. Compatibility

Minitest has better compatibility with a broader range of Ruby versions. Ruby compatibility aside, keeping your project up to date is essential: having framework and libraries updated to their most recent versions is critical. It can be for security reasons or for performance benefits. Anyway, newer versions of libraries should be more mature, have more bugs fixed, and are usually better performing.

To have a nice update cadence, you need good test coverage. You have to know if your code is still working as expected after updating or upgrading the library. I saw many projects stuck with Rails couple of versions behind the main. A team struggling with upgrading the Rails version is usually an unpleasant sight to see.

There are 2 main obstacles preventing people from regularly upgrading:

1. There is not enough coverage. You just do not know if you can safely upgrade without breaking something
2. 3rd party code or library is incompatible with the most recent version of Ruby or Rails

When it comes to the second point, RSpec is sometimes causing compatibility issues once the most recent version of Rails is released. It takes some time for the RSpec team to catch up and get up to speed to release support for the most current version of Rails. Being a standard part of Rails distribution as a default testing library, minitest has no such issues.

4. Rails team is using it

For so many different reasons, it is essential to stick to the conventions the Rails team is using themselves (I wrote about this before). I am a big proponent and a follower of the "convention over configuration" principle. It is already there; nothing is required to be installed in addition. Minitest is the default and assumed testing framework for the new Rails applications. If you believe in Rails philosophy and its way of doing things, that should be enough reason alone.