Immature seeds are whiter in color. I say the darker and bigger, the better. Instead of hanging buds that have seeds, I usually just cut the buds off and put in a paper bag to dry. This way the seeds don't fall off the drying plant and get lost. When the bud is dry, the seeds are probably dry and good to go also. While the buds from a seeded plant aren't as good, they aren't bad either. Don't throw it out.
With a seeded plant, it can take longer to mature. Ideally you want to see many seed pods popping open with visible dark seeds before you harvest. It can take awhile longer to finish.