The beaches are fairly comparable -- after all, it's the same ocean and the areas are only about 7 miles apart. The two neighborhoods have a different feel to them:
1. Manhattan Beach is a "beach town" -- you won't see too many tall buildings or major shopping -- just houses and shops crammed together on a little bit of a hillside (easy to walk up and down) overlooking the ocean.
2. Venice/Marina del Rey has more of a "city feel" While there are smaller homes here, there are also high rise hotels and office buildings.
I wouldn't let the lack of a free hotel shuttle stand in your way to either of these destinations because you have some options:
a. A taxi will run about $20 (Come on, think of the price you paid for your plane ticket. What's $20 to have a great time at the beach?)
b. You can take a bus. This is either free or really cheap but it takes longer than a taxi and you have to transfer out of the airport (via a free shuttle) to pick up a city bus.
i. To the Foghorn, take the free shuttle bus to PARKING LOT C that will pull up in front of your terminal under the RED SIGN that says HOTEL AND PARKING LOT SHUTTLE.
Get off the bus at the first stop in the parking lot, and walk out to the sidewalk and turn left. The bus depot will be a very short walk down the sidewalk on your left. Get on the "Santa Monica #3" bus marked "LINCOLN BL-MONTANA-UCLA" heading east. The fare is 75 cents but request a transfer and you will be charged $1 and given a ticket for the next bus ride. Get off the bus 14 minutes later at Lincoln Bl & Mindanao Way.
From the exact same stop, take the Metro Bus 108 marked "MARINA DEL REY" heading west. Get off 4 minutes later at the corner of Admiralty Way and Via Marina (adjacent to your hotel).
ii. To the Seaview, take the free "G" shuttle bus under the BLUE overhead sign in front of your terminal. The sign will read: "LAX Shuttle & Airline Connections". Request a free transfer to the next bus (the Beach Cities Transit #109 bus). Get off the shuttle at Aviation Station (the only stop outside of the airport) and board the #109 bus (show the transfer ticket for free admission). 15 minutes later get off at the corner of Highland and Rosecrans and walk south on Highland to your hotel.