LAX Marriott is OK
I used to stay at the LAX Marriott before/after flights rather often. It is an older Marriott property and the rooms are rather small. (And in my opinion, have the smallest hotel bathrooms I have ever seen!) But it is convenient and their shuttle vans run around the clock to the airport. Not sure how "generous" they are with MR Plat uprades. It has been my experience that most Marriotts are not that "Upgrade Friendly".
As for other Marriott properties, there is a Courtyard a block or so closer to the airport. I have never stayed there, however.
Enjoy your stay!