No advantage really. It was the first one I used and just didn't get rid of it. It's purely an automated upvote service so you don't get the trails and other advantages of steemauto. But, when Steem Auto decided not to get their shit in order after the last hard fork and left people hanging for days on end, plus a few other times they did the same, steemdunk was back up and running nearly instantly. Haven't noticed issues with steemauto for a while now so they seem to be much more reliable now.