Having been to both parks within the past 5 years,
Disneyland is my recommendation.
We went to DL every other summer until 1998, loved it every time. I then relocated to New England and going to DL was harder than WDW, so have gone there twice.
We went in 2002 during April vacation; it was very hot and humid but no rain. Our last trip to WDW took place last year; Orlando is awful, especially if you hate heat and humidity; we were there last Labor Day for 5 days and we skated the edge of dehydration daily and that was with water bottles and a camelback water carrier! We stayed on-site both times.
Personally I am never going back to WDW; it just doesn't have the magic that DL has, I was very disappointed with both experiences; the parks are huge but there arent anymore rides than DL (although I loved Animal Kingdom). You walk a lot; but there is a lot of nice scenery and picture taking opportunities. Its far too hard to eat off-site, all meals take place at the resorts or in the parks, and I found the food only adequate in most places, it all started tasting the same. Well, except OHanas, that was incredible!
Getting back to the resort during the day for a break is hard; its a good half hour transit each way depending on where you stay and if the shuttles are running on time. Leaving at night was terrible, some nights we waited over an hour and a half for a shuttle to our hotel as the lines were so long; it was miserable! We even left before the last fireworks one night just so we could get in line early but still ended up waiting 45 minutes.
Because of the heat, all air-conditioned buildings are packed constantly, finding a table for any meal took a long time, plus having people holding their trays and hovering around you as you eat is uncomfortable especially if you want to linger over your meal. At times we had people asking us to hurry as they wanted our table, we werent even done eating, it was BAD!
Park Hopping at WDW is a true hassle; it takes a while to get between parks. Each time we tried to hop it took between 30-45 minutes as we had to wait for a shuttle and the lines were long. Park Hopping at DL and CA are easy, its a short walk, maybe a 5 minute stroll.
DL is just much easier to get around; its easier to take breaks during the day, or go off-site for a meal. I have found the whole atmosphere quite different than WDW; nicer, more polite, and it sounds strange, but more authentic. I felt like WDW was a knock-off.
August in LA is hot, but its a dry heat and it cools off at night, which didnt happen in FL.
Staying off-site is MUCH easier at DL than in WDW; and cheaper too. If you have an Entertainment book you could end up getting a great deal on a hotel. We are going to DL in September; we are staying at HoJos, 2 premium rooms at $69 a night for 8 nights because of the Entertainment Book rate. The savings are going right into Character dinners and breakfasts at DL and dinner at the Blue Bayou at least once. If you were planning on 10 days at WDW, 10 days at DL would be ideal!
Personally, PotC, Splash Mountain, and Space Mountain are better at DL, and they have the Indiana Jones ride, WDW doesnt.
Many people prefer WDW to DL, it all depends on how you like your heat!
Good luck with your trip! Have a great time planning it.
