We wrestled with the exact question ourselves. We stayed at Aulani as part of a two week visit to Hawaii which included a hop over to Maui and two nights in Waikiki prior to leaving. It was a combined
DVC points and Marriott Rewards points vacation since I used to travel 120 plus days a year. While at the Waikiki Marriott (great property btw) we were offered a comeback visit at the MVC for $399 for five nights the only requirement was to attend a 1 1/2 sales presentation. We could also upgrade to a 1 bedroom for another $150. We fell in love with HI and the thought of coming back was easy.
So last summer we made our second trek out to Hawaii stayed at the Marriott Ko Olina Beach Club first then Aulani. Our intent was to check out Marriott and beat the system with an affordable stay. The sales presentation is on par with the DVC presentation, it was professional and low pressure. Marriott is a quality product and operates much like DVC with a point system that was introduced about 4 years ago(although some older owners prefer the weeks system).
In the end we decide to purchase at Marriott rather than add points to our DVC membership. It was a tough decision as we love our DVC membership and use it extensively. Ultimately, it came down to a financial one, we've seen over a 60% increase in DVC points since we first purchased. Also, we wanted to diversify our vacation options as the kids get older (they are now 12 and 8). We are thinking we can alternate years between DVC and MVC vacation and effectively double our points for each program.
Ultimately, I feel both a quality programs. They have different target audiences and each has its own pros and cons. The major difference I see, is that with MVC there is no Home Resort so everyone can book at 11 months. It also looks like you can book further out than 11 months at a higher point rate. As a Disney fan boy there is no way Marriott is going to measure up to DVC so it isn't fair to compare apples to apples. I've added Marriott VC to diversify our vacation options. We do intend to visit HI again and probably a few more times, I see us either staying at Oahu and using MVC for our base for sightseeing whereas Aulani is its own destination or staying at another island with MVC and Aulani for Oahu.
It's really hard to compare a higher end resort like Marriott with an immersively themed resort like Aulani, Aulani will win every time. I will say just comparing the two resorts, Marriott had a bigger 1 bedroom villa and the kitchen was better equipped (i.e. utensils, cookware, etc). I don't think there was any comparison between Auntie's Beach house and the recreation center at Marriott. DVC also has a better pool and slides for kids. Both had ample actives scheduled throughout the day.
Feel free to PM me if you have any questions. In the end, I don't think you will go wrong with either.