I'm in both systems, have stayed at MVC Ko Olina and have toured Aulani. DVC is a better / higher-end system to me, but it is limited. For DVC, you or at least we basically have 3 places to go in my mind -
Disneyland area, Aulani, and Walt Disney World. I don't see us taking a vacation in Vero Beach, and if I want to go to Hilton Head to play golf, I probably will use my Marriott. The Marriott is more useful as there are far more places to visit and many more options on room type. DVC offers far more memorable options.
For Ko Olina specifically, the Marriott has bigger rooms that are more generic. The restaurants at the Marriott are better. Many of the activities that are available at the lagoon in front of Aulani are available at a nearby lagoon to the Marriott. The big advantage to me for Aulani is access to the snorkel area which is only available to Aulani guests. The lazy river looks great also, and if the water temperature is good, that is clearly something the Marriott doesn't have. Lastly, it is much easier to walk to the mall across the street from Aulani versus the Marriott.
Given that I have two kids and mostly just book 2 bedrooms, the Marriott system is more economical, but the really cool lodging options are definitely with DVC so I have both and am happy with both. For example, on a recent trip to WDW, we stayed at MVC Sabal Palms for half the time and DVC Animal Kingdom Safari view for the other half -- both with 2 bedrooms. Sabal was comfortable and quiet, but DVC Animal Kingdom was borderline mind-blowing for my kids -- and I loved waking up to seeing a few giraffes every day!