If all the Parks were in full operation then certainly California is worth the trip. Disneyland is the original and is a charming place to visit. It also boasts better versions ( in my opinion) of a number of the classic rides.
DCA I can't comment on as it's undergoing major development which one hopes is for the better.
Universal Hollywood and Seaworld San Diego are smaller versions of the Orlando Parks but don't suffer for that at all. They both have their own personality and charm too.
Knott's , which I always liked, appears to have fallen a little behind in recent times and may prove the only disappointment.
Harry Potter debuts at Islands of Adventure in Universal Orlando this June, so if your trip can't wait for a year or two, Orlando might be a better option.
Within the next two years, like James mentioned, Disney's California Adventure will boast many new attractions and themes. Robert Niles, the site editor, posted an update on DCA today. Plus, Transformers and King Kong at Universal Studios Hollywood should be complete.
I now usually make my trips to Orlando (I live in L.A.) in spring and fall. The fall is best, because you can do both Halloween Horror Nights at Universal AND Food and Wine at Epcot...(and by October, the Harry Potter land will have ironed out the kinks).
Southern California is definitely worth the trip, though -- although you might be better to wait until the DCA construction is done in 2012. With two Disney parks, one Universal theme park (which, in my opinion, is their best individual park), a SeaWorld and a Knott's Berry farm, Southern California is truly a FANTASTIC theme-park lover's trip. And, you can throw in Six Flag's Magic Mountain -- home of a dozen thrill coasters -- plus Hollywood, etc. In 2012, the new DCA is really going to be a great park. I can't wait to experience the new Cars Land -- it will be a one-of-a-kind place, so you can only experience it in Anaheim. Same thing for the Indiana Jones thrill ride in Magic Kingdom: it's only in Anaheim. And that goes for the California Screamin' roller coaster (better than Rock N Roller Coaster, in my opinion, because it's awesome the entire time instead of just the first half). The new World of Color mega-show debuts this month (soft openings have been huge successes) at DCA...and the list goes on and on.
Hollywood's Universal Studios boasts the largest working movie backlot in the world -- almost worth the trip all by itself. And Hollywood's Universal theme park has some of the elements from Florida's IOA, including the Jurassic Park ride. In my opinion, the Universal Studios theme park in Hollywood (as a single park) is better than the single park of Universal Studios Orlando (I'm not including IOA in this assessment). that being said, the double-park combo in Orlando is definitely a treat (although the studios part of that combo is rather boring to me -- but I haven't ridden their new Rip, Roaring Roller coaster yet).
Ah, I love theme parks. And I love both Orlando and Southern California....both of which will soon have Lego Lands!
Hope this helps....
P.S. Orlando theme hotels are better -- although the Grand Californian in Anaheim is great, especially since it gives you access directly into DCA.
If you are a big Disney fan, Fantasyland will be mostly under construction at WDW. Disneyland in California is completely open and really something everybody should expereince even if you have been to WDW
If you want Harry Potter, then go to Flordia.
From your paragraph, I would say Orlando is better for you right now, but go to California in 2011 perhaps.
BTW, I am going to Califoria this year!
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood