I have found from experience that the speed is best chosen by the program you are you using. There are so many differnt variables, such as program and make and type of DVD/CD burner. In older DVD units it worked better at slower speeds due to the buffer getting full(some older models had no buffer at all) and not being able to keep up with the data causing an error called buffer underrun. Since having a modern machine i have not had a problem at higher speed transfers and would say that the program normally decides due to knowing what you have. Hope this helps.
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