When is your trip? San Diego gets June gloom the first two weeks of June. If I were to choose, I'd choose L.A.

Who are you traveling with and from where are you coming? LA and SD are quite different. My kids (the teenagers) prefer LA.

My teens preferred San Diego and the surrounding area. though Hermosa Beach would be a good spot if you do decide on LA. You will need to plan out any adventure to avoid the traffic tie ups, if you end up in LA. SD has traffic, but it seems more predictable and confined to a smaller area than LA.

We get June Gloom in coastal L.A. County too. In fact, we also had "June Gloom" all last week during Spring Break.

Wow what an awesome response. It is 2 adults, we live in PA, we are coming mid to late May. What about a harbor excursion, any recommendations? My son is in LA for the summer. Would it make sense to take the train from LA to SD, rent a car in SD and then fly our of SD instead of renting a car at LAX and going to SD then returning to LAX. Other than one more chance to see my son, I wonder if its worth going back. Thanks again.

The weather in mid to late May is usually excellent, but no guarantees. There is virtually no difference between weather in coastal San Diego and coastal LA, such as Hermosa Beach.
Taking the Catalina Express out of Long Beach or San Pedro, or the Catalina Flyer out of Newport would be a great day trip. There are also harbor cruises out of both places.