So, since I've been obsessively searching for London hotels today, I thought I would also get out my app and see if I could spot differences between the points rates on the app vs. website. And yes, I absolutely can. However, I think no matter which you use you will end up at the same rate.
The app doesn't seem to display the "PointSavers" rates, and displays the usual points required. If you use the Marriott site on your computer, they have some of the points prices in green text and are labelled Points Saver and are discounted rates.
One example is December 11-12, London Marriott Hotel Marble arch. On the app, it says the points are 69,000 for a night, but when you click to book it shows the points required are 55,200. However, when you use the Marriott site on your computer it correctly shows the PointSavers rate of 55,200.
That said, even the Marriott site is deceptive, sometimes those points saver rates advertised are not to book a night, but instead are points required PLUS cash.
Knowing this, I'm never going to look at the app first and will make sure I click on each of the hotels I'm interested to make sure the points required are for the whole stay and I'm not expected to add cash to the points shown.
What Marriott.com shows when you search:
What the app shows: