The question:
Is a Symfony 3.4 project created with the Flex installer really Symfony 3.4? Or more a "Symfony 4 project with 3.4 core files?
The complete story:
I am working on migrating an existing Symfony 2.8 project to Symfony 3.4. At first I created a new Symfony 3.4 project using the Symfony installer:
symfony new MyProject 3.4
The file and folder structure is a bit different to Symfony 2.8 but overall everything is quite similar. It was not too difficult to migrate the config, files, etc. from the existing 2.8 project.
However, after some more research I thought it might be a good idea to start the new 3.4 project using the recommended Flex installer instead. This would give me the new folder structure which is also used in Symfony 4, which might make a future migration to this version easier.
composer create-project symfony/skeleton:3.4.* MyProject
Of course the directory structure is different in this project. But not only the folders are different but almost everything. The way the complete config is organized, how the routes are defined and managed, how bundles are organized (none at all), how parameters are defined and used, etc.
There is almost nothing left that looks like Symfony 3.4. This seems to be a Symfony 4 setup where only the files in vendor/symfony/... are from version 3.4
Migrating the 2.8 project to this setup would be way more complex.
Of course I know that a lot has changed in Symfony 4 while Symfony 3.4 is still quite similar to Symfony 2.8. But how is it possible that two Symfony 3.4 installations are so dramatically different?
I understood that Flex is a new workflow to manage projects but shouldn't be the end result - a new Symfony 3.4 project - be more or less the same no matter which workflow / install was used to create it?
Flex is more powerful when setting up and managing different project. However I want to setup the project once and run it for several years. Is there any advantage (in the near future) in proceeding with the Flex approach?
To answer your first question: Yes, a Symfony 3.4 project created using Flex is indeed a 3.4 project. "bin/console --version" will verify this.
As far as the directory structure goes, Symfony tries to strike a balance between maintaining backwards compatibility but at the same time keep moving forward and refining their design. For information on why things were done, it is useful to keep an eye in the Symfony blog and perhaps visit the slack channel. Especially the posts when Flex was introduced as well as the new directory structure.
One goal for Symfony is to make it as easy as possible to migrate from a long term support version to a newer version. Hence 2.8 is pretty much identical to 3.0 except that a bunch of depreciated functionality was removed. If you were following things at the time you would have noticed that there was quite a big jump between 2.7 and 2.8.
Same for 3.4 to 4.0 being easy but a few possible bumps going from 3.0 to 3.4.
Kind of meandered a bit. Sorry.
To address your second question of the value of using Flex, like all opinion based questions the only correct answer is: it depends.
In my case the choice to use the new directory structure was easy. I have a number of apps and use Symfony on a daily basis. Keeping up with the latest and greatest makes sense for me. Spending the time to port apps (some of them dating back to 2.3) to 4.0 makes my overall maintenance burden easier. The time spent experimenting how to port helped me understand the new functionality. Once I got comfortable with the process it was really just copy/paste/rename exercise for the most part. The hardest was making sure some of the 3rd party dependencies were properly updated.
In your case, it seem like you only have one app and you just want to get it updated and then forget about it for a few years. In that case, keep the old structure and move on. At some point you might need to move to the new structure but waiting a few years won't hurt anything. And hey, I suspect 5.0 will introduce more changes.
The choice is yours.