Admittedly, IANAL, and I don't know much about how foundations (Stiftungen) work in German law, but from experience, e.V. has the advantages of being relatively easy to set up and maintain, being super common so lots of people have experience with it, and having the option of becoming tax-exempt if you put the right weasel words into the statute and file some paperwork with the tax office.
I'm not really sure what the overall conclusion is except that choosing a suitable legal structure is not easy.
> tax-exempt if you put the right weasel words into the statute
The law lists a finite amount of ways [1] in which an e.V. can act in the public interest ("Gemeinnützigkeit") and thus become tax-exempt. Most of these items are engineered to cover existing e.V. For instance, sports clubs are explicitly covered, as are consumer protection watchdogs.
For any associations doing charitable work in the IT context, the issue is that none of these bullet points in the legal text explicitly cover such activities. There has been some support for the idea of adding free software as a charitable cause to this list, but nothing has come out of it as of yet.
I'm familiar with this topic because I'm volunteering in the Chaos Computer Club. Most local chapters are constituted as e.V. and are tax-exempt under the purpose of public education ("Förderung der Volksbildung") since that one is the easiest to get away with if you do any sort of public events that can be considered education every once in a while (like conferences or talks or such).
