That's easy- DTD. It has interesting stores to browse through and fun "street music" in the evening. Gardenwalk has mostly clothing stores. I found GW for the most part to be dull and uninteresting if you are not looking for shoes, sunglasses or clothing. GW is good for places to eat. We like Bubba Gumps there. The shops there are not worth the time or the walk.
I voted DTD. The only reason I would consider Garden Walk over DTD is if you are staying next to Garden Walk. Othewise DTD is a lot more fun to explore.