I've used both SW and DU.
I booked my first cruise with SW and they were AWESOME! Right before that cruise, the agent I used decided to retire. On a whim, I thought I'd use DU and had a great experience there as well.
I stayed with DU for my next 4 cruises...but things slowly went downhill. Tiny issues kept popping up and the attitude in response wasn't great.
So this time around, I decided to give SW another shot and have been nothing but pleased! My new agent has been phenomenal! I highly recommend.
Cheers
Josh