For such a short overnight stay, I suspect points earning and fripperies are of no great consequence.
I've stayed at most hotels on or just off West Century Blvd and find them much of a muchness. I've booked the Westin LAX with priceline.com for $75 and $74 for my next US trip. It's a little further down the road than opalfruit's suggestions but in the shuttle bus, what's the difference.
BTW, there was a similar discussion recently but I can't find it using Search.
Can't advise re Santa Monica and the Marathon.