Occasionally I get questions about the advantages and disadvantages of balanced vs. unbalanced second stages. I thought I would post a few comments here not just because I'm lazy and pointing to a thread is easy, but also to open the discussion for other's insights.
When a second stage is mated with a balanced first stage such as a MK 5/6/7/8/9/10 it will make very little difference in performance with either a balanced or unbalanced second stage. Performance-wise, the advantage of a balanced second stage is realized when it is mated with an unbalanced first stage. (MK 2/3/200) The balanced second can compensate somewhat for the lower IP an unbalanced first stage will deliver at lower tank pressures.
Performance aside, here are some of the pluses and minuses of each.
Balanced second stage
Holds tune longer.
Can accommodate a wider variety of intermediate pressures.
The seat last longer (due to a lighter spring pressure when not in use.)
Unbalanced second stage
Seats are less expensive and can be flipped over and used on both sides.
Fewer o-rings makes for a (marginally) cheaper service.
I'm sure there are other ideas out there; let's hear them.
BTW Service kits for balanced and unbalanced 109s are available from VDH