I have never been to DL (planning a trip for June 2013!), but have been to WDW many times. With such a short amount of time, realize that you won't get to see near everything. Are you staying on property? If not, there will be commute time to consider. And very little at WDW is within walking distance of each other. All that said:
Epcot is one of my favorite parks! I love World Showcase. I love to look in the gift shops, catch the movies in Canada, France, and China. There are street performers in the United Kingdom that are so fun! Plus, there are bands at the UK, Canada, and Moracco. There is a ride in Norway and in Mexico. My favorite counter service restaurant in all of WDW is in France - their bakery is delish! And my one of my favorite table service restaurants is Le Cellier in Canada. The mushroom filet is wonderful! Don't miss Illuminations at night. My favorite ride at Epcot is Soarin', which I'm sure you've already seen the original. It may not be worth the lines for you. I always ride Spaceship Earth and it's rarely crowded. It's a slow-moving ride that chronicles communication. Not a headliner, but still cool to go up into the "golf ball."
I'm sure you are just as intrigued by the differences in WDW's MK as I am in
Disneyland. However, if you only have time to go to two parks, I would suggest going to Animal Kingdom. It has the Kilimanjaro Safari, Expedition Everest (my FAV Disney rollercoaster), and two great shows in Nemo and the Lion King. That park does close earlier, so you might could spend 4-5 hours there and then hop to MK for the last 4-5 hours of that day. There are buses that will take you from AK to other parks. I'm not sure if the bus goes all the way to the MK bus stop or if it will take you to the Ticket and Transportation Center (TTC) where you will then either take a ferry or monorail to the MK.
If you are planning to eat at table service restaurants on property, I would suggest you make reservations (ADR - Advanced Dining Reservations) NOW. You can make ADR up to 6 months in advance for table service restaurants. I know that seems ridiculous, but many of the popular places fill up quickly. One spring break, I saw at least 20 parties turned away at the table service in France at Epcot while I was waiting to be seated. You can make reservations by phone or online. Allears.net is a great website that has all of the restaurants at WDW and their menus listed. While I don't use that to pick out exactly what I'm going to eat in 6 months, it gives me an idea of whether or not I'll like that restaurant.
Stay hydrated. I don't know how the weather is in southern California in the summers, as I've not been there yet. However, in Florida it's hot and humid. The humidity will make you feel suffocated and have you sweating as soon as you walk out the door in the morning. Often in the summer, there isn't even a break in the humidity at night. Just be prepared and drink a lot of water.
I hope this is helpful. Enjoy your first trip to WDW! Maybe you'll see enough on this trip to make you want to come back another time!!