September is a good time for visiting the coast. The weather is usually clear and dry, and the summer crowds are gone. You shouldn't have much trouble with crowds at the beach, whichever one you choose. And if you go during the week, there will be even fewer people.
I think the reason you're getting mixed reviews is because the beaches are so different, and they appeal to different people.
Malibu is essentially a long narrow strip of land nestled up against the Santa Monica Mountains. There are some very scenic beaches here, with varying amenities. Some are small pocket beaches with little more than a stairway to the beach and a chemical toilet. Others are long sandy beaches with plenty of amenities. Many have a fee parking lot, but for some the only parking is along the shoulder of a road.
This is the place to go if you're looking for a quieter beach. El Matador is noted for its scenery, if you don't mind stairs and minimal facilities. Point Dume offers great views and nature trails. Leo Carillo, Zuma, Malibu Lagoon, and Will Rogers have plenty of facilities.
In contrast, Santa Monica has one very long wide beach, Santa Monica State Beach (which is operated by the city of Santa Monica). If you're looking for fun, this is the place to go, because it has the most variety of activities. At the Pier, there's an amusement park, an aquarium, restaurants and shops. You can rent skates, bikes, or surfboards, and Lifeguards are on duty year round. The city rakes the sand and cleans the restrooms daily.
You can also check up on beach cleanliness at Heal the Bay
which issues beach report cards
on the water quality at over 500 beaches.
I don't have detailed descriptions of the various beaches, but I do have a map with brief descriptions, which may help you out. You can switch to satellite view to get an idea of the terrain. Zoom in to get an idea of what the beach looks like. If you want to see it larger in a new window, click on the link below the map.View Malibu and Santa Monica Beaches in a larger map