The weather in April in SF is fantastic. One of the better months all year to visit. And, I think a great destination for families, with many of the activities being low cost or free. Fisherman's Wharf, the Exploratorium, the museums in Golden Gate Park, and the bay itself are all great choices for kids and reasonably priced. If you enjoy outdoor activities, there is incredible hiking just across the Golden Gate Bridge in Marin County...totally free.
LA is glitzier, but frankly, harder to get around...and activites can be quite spread out.
There are very different from one another...you really need to think about what it is you want to do, and how you like to vacation.