South Mission Beach is quieter. MB is a rental community now lots of kids from SDSU & Mesa living there. More sunbathing, biking, beers and margarita crowd.
I would try to go bayside south MB not on the alley. Then you can walk up the boardwalk to ride the wooden coaster and its a quick out to the 5 & the 8 for Sea World, going over to Coronado etc.. Cairncross realty, Jeff does rentals and I think Penny realty still rents weekly.
La Jolla is more shopping, martinis, fine dining. Rentals are more expensive & likely cottages or condos for familys. Depending on how far into LJ you are, you can hit traffic to get to major roads. We like the Cove Suites.