It's hard to say which is best in general, because in each of these pairings, the pairing has value--you always learn something from whomever you're pairing with. Most of all, I think the best combination depends on the specific individuals in question. Some feel more at home pairing with someone more senior, someone else is more comfortable with an equally experienced peer, and so forth.
Novice - Novice is a little bit risky. Not as risky as having a novice work alone, of course.
Experienced - Novice will certainly help a lot getting the Novice up to speed. Of course the experienced developer will be very much in a mentoring role, which not everyone likes to do for much of his time.
Experienced - Experienced can be *very* productive, in my experience. With some practice, communication is a non-issue, we understand immediately what our partner is getting at with a new idea, often before he even finished explaining it. There is a lot of mutual inspiration to gain.
Why do you ask?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus