To start your project on Twitch/Curseforge, here is the link:
https://wow.curseforge.com/ and all you need to do is click the button for Start Your Project.
Once there, you have a choice (last time I checked, anyway) of uploading zip files or using the repository system. Curseforge supports Git, Mercurial (Hg), and SVN. It can also pull from external repositories like GitHub. You will also have access to their localization system (you have to turn it on per project) and let users submit translations. If you are using a repository system, it will automatically grab the latest localizations and build your zip for you. If you are manually uploading zip files, do not forget to export the latest translations and update your files. That is not automatically done for you. Finally, Curseforge offers Author Reward Points (opt-in) where you can collect points based on downloads. You can trade those in for various rewards.
Wowinterface does support repositories, but other than SVN, I am not sure which ones. For authors, it is the second largest collection of addons, many of which are in both Wowinterface and Curseforge, but as noted by Rilgamon, it is a good idea to upload in both places. Wowinterface does have a couple of methods to pull files from Curseforge and hosts copies here. You will need to manually update your changelog if you do that, but that is minor. And finally, for authors, the forums here on Wowinterface are much more frequented by your fellow authors along with users. Curseforge does have forums, but they are not as active. Oh, and I did forget that Wowinterface supports manual zip uploads as well.
Hopefully all that helps.