I would prefer a day per park but if I only had one day, I would just do Disneyland and skip California Adventure. Or if you get a park hopper, just worry about Cars Land and Guardians so you can see how that differs from Tower of Terror. If you get Genie+, it's a breeze to blow through both parks. Just get there at rope drop so you can hit up rides with low waits. If you only want to worry about differences, get a park hopper and only hit up the exclusives and maybe must do favorites and then park hop to CA. You can always go back to DLR in the evenings/when done. I think you won't get to do Haunted Mansion as that's down until the fall. The Splash Mountain changeover may be done by then. I think they were angling for late summer. Definitely get there early in the morning and be amazed at how much you can get done before 11am. Genie+ is superior at DLR and CA because less people use it. You can work it so you don't have to use Genie+ till around 11am. Makes it nice to have those for later so you can ride again if you want.
Guy on a different site made this list which I agree with:
DL Exclusive Attractions:
Indiana Jones and the Temple of the Forbidden Eye (uses same ride system as DINOSAUR at AK)
Snow White's Enchanted Wish
Pinocchio's Daring Journey
Mister Toad's Wild Ride
Casey Jr. Circus Train
Storybook Land Canal Boats
Alice in Wonderland
Sleeping Beauty Castle Walkthrough
Roger Rabbit's Car-Toon Spin
Chip & Dale's Gadget Coaster
Finding Nemo Submarine Voyage
The Matterhorn (note this uses the same ride system as Space Mountain at MK)
Space Mountain (same theme but completely different ride)
DL Similar but not exactly the same:
Enchanted Tiki Room - longer show
Pirates of the Caribbean - longer with more show scenes
Haunted Mansion - even without the Nightmare overlay it is slightly different
The Many Adventures of Winnie the Pooh
Big Thunder Mountain - some additional effects and scenery
Autopia - much longer than the Tomorrowland Speedway, but basically the same
Buzz Lightyear Astro Blasters - removable laser blasters make it a little better
DL Exactly (or basically) the same:
Peter Pan's Flight - technically some minor differences
Dumbo
King Aruthur Carousel
Milennium Falcon: Smugglers Run - all of Galaxy's Edge is practically identical
Rise of the Resistance
Mad Tea Party
Mickey and Minnie's Runaway Railway - The queue is very different and cool - may be new to you and is worth it if so.
Star Tours
Astro Orbitor - though at ground level
CA Exclusive Attractions:
Monsters Inc. Mike & Sully to the Rescue
Guardians of the Galaxy: Mission Breakout - based on Tower of Terror, though the DL version works a little differently
WEB Slingers
Radiator Springs Racers - this is like a massively bigger Test Track
Luigi's Rollicking Roadsters
Incredicoaster
Inside-Out Emotional Whirlwind
Silly Symphony Swings - though this is an off-the shelf "wave swinger" ride
Jumpin' Jellyfish
Golden Zephyr
Goofy's Sky School
CA Similar but not exactly the same:
Mater's Junkyard Jamboree - the same as Alien Swirling Saucers
Grizzly River Run - longer version of Kali River Rapids - definitely a wet ride!
CA Exactly (or basically) the same:
Mickey's Philharmagic! - smaller screen though
Turtle Talk with Crush
Ariel's Undersea Adventure - same as Under the Sea: Journey of the Little Mermaid
Jessie's Critter Carousel
Soarin' - though note sometimes they run it with the original Sorain' over California, in which case it is a must do!