Desert Willows is more modern and the resort has a more mediterranean feel. Westin Mission Hills is quite a bit older and while we love the golf facility, we are actually not crazy about staying in the units. We just stayed in Dec 2017 and Jan 2018 and they were quite worn and the colors within the unit were dark brown and dreary. Practice facility at Westin Mission Hills Golf Course is free, and with great short clinics each day for $25 per person per day. Westin Mission Hills hotel which is adjacent to the timeshare resort is a plus compared with Desert Willow, with additional restaurant options.
We love Marriott Desert Springs I and II but they don't have clinics like at Westin Mission Hills. The refurbishments at Desert Springs have been fabulous. The units are cheerful and tastefully done and the units are bright. There is high ceiling at the dining area at Desert Springs II, with electric switches against the wall to operate the drapes. Very nice touch. Some other examples here: When you get out of bed, there is some sort of motion sensor that comes out at the bottom of the bedside table. In addition to lights on each side of the bed, there are also separate reading lights. There are many more things that we have been impressed with well-thought out touches they have done with the resort.
Desert Springs is also next to JW Marriott which has many more and better restaurant options compared to Westin Mission Hills.
We normally split our stay when we are in the area, which is usually twice a year, a week at Westin Mission Hills for the golf facility and a week at Desert Springs to enjoy the resort. We golf both weeks.