DME is used alone without other sugar, and, honestly, it doesn't make any difference. Some people are purists and think that a little sugar will harm their beer. It just aint so. In my experience, corn sugar is a much faster, cleaner priming sugar and contributes zero taste to the beer in the quantities used for priming. DME will definitely take longer to condition and you use a bit more of it usually. So, no real difference except corn sugar works better for the purpose.