There is too much in both parks to do everything in one day. I think 2-3 days is best, however that may not be an option. So if 1 day is all you have, pick and choose what you see.
Since you're a regular WDW visitor, do some research between what both parks share. For instance, as much as I love Toy Story Mania, Tower of Terror, and Soarin, I can't recommend them to a WDW regular with 1 day only because they're very similar to the WDW versions.
At the same time, do not assume that just because both parks share a ride of the same name, the rides are the same. Pirates of the Carribean is far superior at Disneyland. The two do not compare. Likewise, Space Mountain is a completely different experience both ridewise and audio experience in Disneyland.
I can't back up Hound's tips enough. Be there early and have a plan. As Han Solo would say, "flying through hyperspace ain't like dustin' crops." You can knock out most of Fantasyland in the first hour or so if you're there when the park opens. Likewise, Fantasyland empties out the last hour the park is open.
And since World of Color is unique to the DLR, plan on getting a fastpass for it since it is not to be missed.