I would say do the things at DL that they don't have at WDW such as Matterhorn, Mr. Toad's Wild Ride (removed from WDW around 1998), Indiana Jones ... oh, and eat at Rancho del Zocalo, perhaps one of my favorite places at DL.
Generally, I/we like DL much better than DW. It's smaller, yes, but also more dense. There's more stuff there and it has more charm. As others have said (and will likely say again), it's also the original park -- the one that Walt built.
Have a great time!