It depends, every role has different permission and you may choose thinking about the way you're going to use this web-site.
Choose the "artist" role only if you are an inkscape user and you want to publish your work, we check the new users and if you appear to be an artist but you don't post works we will change your account giving another role. This is necessary cause of the personal profile and gallery that only artists should have.
If you choose "forum user" account you will be able to post topic to the italian and english forum, you can send and receive personal meessage but you can't post your inkscape files.
Choosing registered users is the right choice if you want to be able to see alla the content of the web-site but you're not interested in postinf forum topics and inkscape images.
You will be able to post comments, but you will have to wait for approvation before to publish them.