'Stream of Consciousness' is usually considered a narrative mode - it's a style, more than a habit of writing. You could absolutely write in stream of consciousness using an outline. Think of it like intense first person, with no rational filters or structural impositions.
I think what you're asking is 'which is more productive, discovery/'pantsing' or outlining?'. The question has been more-or-less asked and answered, here. The only difference with your question is the idea of productivity, and I'm not sure if that's a useful concept, really. It may be too vague, because... what are you trying to produce?
If you just want a lot of words, pants it. Sit down and write, and see what comes out.
If you want something of good quality, things get trickier. I honestly think this may be a genre specific question, at least to some extent. I write Romance, where the formula is quite rigid, and the challenge comes from trying to make the same basic story structure feel fresh and original. It's possible to write by the seat of my pants when I'm writing Romance because the genre imposes a structure of its own. I can be a discovery writer and let my characters explore their world because I know, at least in the largest sense, where they have to go eventually.
But when I try to add a mystery, with the need for clues, or a thriller with complicated plotlines, I find myself needing more of an outline in order to keep myself on track. Some people can write complicated works without an outline, but I'm not one of them.
I think it also depends how much re-writing you're planning to do. when I write from an outline, I don't need to make serious changes before the book's ready to go, because it was already fairly structured and tight. When I write without an outline, I need to be prepared to go back and chop or add to make things work out.
Honestly, I think this is another area where each writer needs to explore and find out what works for him or her self. Sorry.