I don't see any advantage one way or the other. Some people swear that DME gives finer bubbles, but I think that's silly. Carbonation is carbonation. If you have DME on hand, and would have to buy corn sugar to use it, then use the DME. If you want to use sugar, that's fine. It's really a very small percentage of the beer and it really doesn't matter. I've done both, usually because I don't have corn sugar when I go to bottle.
One thing to be aware of is that with DME, it may take a week or two longer to carb up since the sugar is more fully fermentable. Also, you'd use me DME because of that. So, if you use DME you could use 1 1/4 cups of DME but only 3/4 cups of sugar for the same carbonation level.