This is one of those "ask 10 different people, get 10 different answers" kind of questions, but I always build my wheels with the same guage spokes on both sides, and they are usually DT Competition 14/15/14. Some people like to run thinner spokes on the non-drive side to save weight, and others have some kind of practice based on the drive side spokes requiring more tension. I just like to keep it even for simplicity, plus it hasn't been a problem yet.