Disneyworld is VERY different from Disneyland. It is not one park, but 4 separate parks (with, alas, separate admission to each park, unless you get the parkhopper add-on). I would agree with sticking with one park for a single day trip. There will still be plenty for you to do! As for which park...that really depends on what you and your husband like to do. Single day tickets for MK are slightly more expensive than the other 3 parks.
If you like rides and "mouse magic" I would say probably Magic Kingdom. Get a Times guide and catch the new Festival of Fantasy parade, ride all the "classic" rides, eat at Be our Guest (if you can get a res) and watch Wishes in front of the castle. I tend to think of MK as the "kids" park, but my husband and I want to do the "behind the scenes" Keys to the Kingdom tour without the kids. You *can* see Wishes from various other locations throughout the WDW complex (Fort Wilderness, Polynesian, etc) and they even pipe in the music! MK is usually open the latest, so this may be the biggest "bang for the buck."
You'll also be there during the Epcot Flower and Garden show. Lots of neat topiary displays to see, try new foods at the outdoor kitchens (on top of all the "usual" places at world showcase), flower power concert, and various demonstrations. You can eat/drink your way around the world showcase. Soarin', Test Track, Maelstrom, and Mission Space are fun rides there for adults. This is usually the park we do for a "day trip" without the kids since my husband isn't all that keen on "disney magic."
Animal Kingdom is great for...well, animals. :) But, as others have said, have shorter hours and is a bit more away from the center of Disney. I would maybe save this for a longer trip and use this as sort of a "down" day in between the other parks. Not saying it's not fun, but I probably wouldn't do AK on a single day trip. Another option here is to go to the Wilderness Lodge (maybe a meal at Boma?) and check out the lodge and viewing area in between your park stuff.
Hollywood studios. We usually do this park with our 4 and 2 yr old b/c of the disney jr stuff, but if you and your husband are movie buffs, it could be fun. Fantasmic is an amazing show that's at Hollywood studios. You can do a "package' where you eat lunch or dinner at one of 3 restaurants and get reserve seating for Fantasmic. Indiana Jones, Tower of Terror, and Star Tours are also fun to do here. Oh, and you can learn to draw a disney character at the Animation thing (I forget what it's called).
Don't forget that you can check out the disney resorts, even if you're not staying there. Wilderness Lodge, Fort Wilderness, Polynesian, Beach club (if you like ice cream, check out The Kitchen Sink at Beach Club), etc are neat to walk through and have fun places to eat. And there's also Downtown Disney for shopping and food!
Hope you have a great trip!