As for 'June Gloom' . . . statistically speaking, June is the month with the LEAST amount of sunshine here in So Cal. It's foggy and overcast in the morning, burning off around 2, and then it gets cloudy again around 3 . . .
I grew up here, and it's perfect weather for jeans & a t-shirt with a sweatshirt around your waist for when you need it (unless, of course, you're used to a colder climate).
Don't be fooled by the overcast skies, however . . . sunscreen is VITAL when it's overcast.
I've had my worst sunburns when I couldn't see the sun!
And I know better!
June is also one of the dryest months we have, so I can't imagine rain being an issue. We don't believe in humidity here (seriously - maybe a handful of days per year), so that's not a problem, either. Just check for the Grad Night schedules to make sure you plan around the shortened park hours.
As for Park size . . . if you look at the WDW Magic Kingdom parking lot, both DL and DCA will fit in it!! Walt did not have the luxury of real estate in the 50's so he couldn't spread out like he wanted to, but he certainly cleared up that issue in Florida. We usually describe MK as Disneyland on Silly Putty and all stretched out.
The main place I notice it is Main Street - at DL, the Castle is at the end of Main Street just on the other side of the hub. In WDW, it's as far from the hub as the main gate is. I prefer DL, but maybe because I'm just lazy and would rather not walk for days to get to the other side of the park . . .
I'm sure you'll find things you like better about each park over the other. Looking forward to a trip report to see what you thought!!!