This reply is obviously too late for your stay as you posted this five months ago, but by far, my vote would be for the Marriott Newport Beach Resort. The Irvine Marriott is fine, but oh-so-boring. It is a business hotel mostly. The Westin Costa Mesa is a bit more interesting since it is near Segerstrom Concert Hall/the Orange County Performing Arts Center, South Coast Repertory, and South Coast Plaza as well as The Lab anti-mall, and some nearby non-chain and chain alike restaurants, giving you an OC cultural and shopping experience. The hotel is also fine. But as for the hotel itself, I found the Newport Beach Marriott Hotel and Resort to be the most "exciting" in that the ocean views and golf course are truly lovely, the lounge is much better than many other lounges with good offerings, the spa is nice, there are more suites to which to upgrade, there is a salt-water swimming pool in the spa area (separate from the main pool for families), bike rentals are included in the resort fee (and the beach is just a few blocks away), and it is right next to Fashion Island shopping center. If you are going for the hotel itself, NB wins. If you are going for the area with places to go and things to do, I think Newport Beach and Costa Mesa tie as they are equally interesting, one more than the other depending on your interests. If your interest is to stay in the #1 safest and most master-planned city in America and want to be near the airport, go to Irvine Marriott.