Been to Aulani around 5 times, the Marriott 3 times. I think they are relatively similar - for our family the main differences are no lazy river, no kids club and obviously no Disney references. The rooms are very similar in size, layout and style. Pools are great and lagoons are identical. Pricing - Marriott normally is cheaper.
We have stayed at both Aulani and the Marriott and both have pros and cons.
As mentioned above, the Marriott has no lazy river but the pools are very cool with a waterfall cave you can swim through and a small slide. The pools are also open much later than the ones at Aulani which is a pretty big plus for us. The ground are more spread out and we found there were many more free activities at the Marriott, including premium exercise classes like morning beach front cardio and yoga which Aulani now charges extra for. (you can go to a quickie morning yoga at Aulani but it is really wasnt well done, very basic poses)
There are lots of all day bbqs on the Marriott in two different sections whereas at Aulani you can only use the four that are there in the afternoon until mid-evening.
Is there anyway you could modify or extend your stay to include a night or two at Aulani before or after the Marriott stay?
We split our stay between Aulani and MBC in April. When we decided to go back next week we were torn on where to go. Ultimately we decided Aulani because we are only going for 4 nights and will only have a rental car for one day and we want to be able to walk quickly to dinner at the four seasons and monkey pod and I want to paddle board quite a bit. We love things about both MBC and Aulani. What we like better about MBC is that is is easier to get a chair at the pool, they have the best frozen mai tais, they have fun games (corn hole, etc) in the lawn by the pool and we could rent a cabana at the pool for practically nothing. The pool is a fun pool that feels like its built into a volcano. Lots of fun hot tubs and waterfalls and different levels to the pool. we also like the lagoon a little bit better at the MBC.
What we like about Aulani is the lush gardens, the hawaiian music they pipe in, that you can rent a paddle board, free sand castle toys, and proximity to the Four Seasons and Monkey Pod and ABC store. also the Disney characters etc.
DH and I walked over to visit the Marriott one night during our Aulani stay. As two adults, we appreciated that it was quieter, felt less busy, and it was more "open concept" if that makes sense - less winding, confusing pathways which sometimes causes crowding/congestion during busy times at Aulani.
What's the price difference for your stay? If its significant, I wouldn't hesitate to stay at the Marriott and just visit Aulani. Its accessible by a quick walk along the beachside path. You can use Aulani's beach and lagoon (just not the chairs), you can dine at their restaurants/luau, and you can get a spa treatment there.
Also have stayed at both. What are the ages in your party? With little kids, Aulani is a winner hands down. If it was just me and my husband, we would stay at the Marriott. Love them both. Marriott definitely quieter.