I'd say both, honestly. I wouldn't be surprised Larian staff is going to have a look at the Uservoice forum first and foremost.
While an idea can be more easily discussed here, it can also completely disappear in non-ideas-related posts. At least, while the uservoice is full of ... let's say politely "random" ideas, everything still is Idea. The downside is that you can't seem to be able to edit your initial idea meaning you'd have to post-script it in the comments.
Maybe you can get the best of both world by posting it first, reflect on comments, and when you feel the result is right, post an "enhanced" version of your idea on UserVoice.
