A one day park hopper is adequate if you plan on making it along day. As you said, you've been to WDW before, and are considering making this a side trip. If you do decide to go and only have the one day, then I would make a list of the rides/attractions that you really want to see and do. DL and WDW do have different versions of many rides, and DL does have rides that WDW does not. Here are a few things to consider for the day that you visit DLR.
What rides have fastpass?
DLR: Autopia, Big Thunder Mountain Railroad, Buzz Lightyear Astro Blasters, Indiana Jones Adventure, Roger Rabbit's Car Toon Spin, Space Mountain, Splash Mountain
DCA: California Screamin', Grizzly River Run, Mulholland Madness, Soarin' Over California, The Twilight Zone Tower of Terror
What rides does DL have that WDW does not?
DLR: Matterhorn, Storybookland Canal Boats, Casey Jr. (if you have younger kids otherwise skip it) Mr. Toad's Wild Ride, Pinocchio, Roger Rabbit, Indiana Jones, Alice in Wonderland, Fantasmic!
DCA: Mullhand Madness, Monster's Inc, Grizzly River Run, Ca Sceamin', Aladdin (show),...most of the others are run of the mill, hit or miss...
What rides are different from those at WDW?
Snow White (slightly different), Autopia, Jungle Cruise, Pirates, Splash Mtn, Big Thunder, TSI,
If you only have 1 day at the parks, what rides do you want to go on the most?
Those that are different/not at WDW?
E-Tickets?
Those for families with younger kids?
As many as possible?
If you plan to hit Fantasyland do it first thing in the morning as there are NO fastpasses! Then start getting your FP for the big rides. When the park get's really crowded head over the DCA for a while and hit some of their rides. I would plan to make it a long day, open to close if possible. Also, a mid week day would probably work the best, even with the limited hours. HTH, and have a great trip!