I've had WDW annual passes for 5+ years, and moved to SoCal about two years ago. We go to DL every weekend.
My recommendation:
Go in mid-June or August.
Option 1: Some "June gloom" possible in June. Not all local schools might be out, so crowds might be better in June.
Option 2: August can be hot, though the ocean is warmer in August if you plan to make use of it.
Recommendations for touring:
For days to go do DL, check AP blackout dates. Best if SoCal and SoCal Select passes are blocked out.
Eat a corn dog from the freshly renovated corn dog cart at DL. While at California Adventure, enjoy California Screamin'. Fantastic coaster. Prepare for a long wait to see World of Color, but do it anyway. See the Aladdin show. Enjoy the overall shorter wait times than at WDW. Forget most of your Fast Pass practices. You won't need them most of the time. Enjoy the fact that, unlike at WDW, you can easily walk between the two parks as you wish.
Priceline your stay in Anaheim at a 3 star hotel and rent a car. You can't do SoCal without a car. See the bidding for travel website for strategies.
Don't compare DL to WDW or you will be disappointed. Expect to be disappointed by the castle and the age of the park. Those of us who live here love it, though.
Remember that DL is not in itself a vacation destination like WDW is. WDW has far superior resorts and food. However, SoCal is a real place whereas Florida is fake. People actually live here and there is a lot to do with us "locals".
If you go during June or August and you like baseball, consider hitting an Angels game. Very family friendly.
If going in June or August, head down to Newport Beach. Expect having a difficult time parking, but a fun day nonetheless.
Consider going up to the El Capitan theater in Hollywood to see the new Toy Story movie. Advanced ticket purchases are virtually required. Check out the rest of Hollywood while you are up there.
Consider watching a TV show being taped. See
http://www.tvtix.com/schedule.php for details. Watch out for age restrictions.
If you like animals, go to the San Diego Zoo or Wild Animal Park.
Eat an In-N-Out Burger (double double with fries and a shake is required).
Relax, hopefully enjoy some fantastic weather, feel the ocean breezes on your face, and meet the locals. It's a cool place!
Good luck!