I'm a WDW vet and with you being a regular WDW visitor, I would strongly recommend the hoppers. To us, DL/CA just feels like one big park. Walking between DL and CA is shorter than walking out of the gate of a WDW park to your resort bus stop. We have actually literally walked from the back corner of DL, out one gate and in the other, and gotten almost to the back area of CA in about 10 minutes. We go back and forth all day between the parks and find it is the best use of our time. To give you an idea, DL and CA together (with Carsland added) are a total of 152 acres. Epcot at WDW is 305 acres. Magic Kingdom is 107 acres. Animal Kingdom is 400 acres. Hollywood Studios is 135 acres. So you can see in terms of ground to walk and areas to cover, doing DL and CA in one day is not much more than MK or HS, and a whole lot less than Epcot or AK. The difference is that DL/CA really packs in the attractions where they are more spread out at WDW.
The question I have is whether you are going to try to see Fantasmic or World of Color. I assume World of Color will be running the days you are there, I don't know about Fantasmic. Those two shows are great (we much prefer the DL version of Fantasmic to WDW- love the ship with Captain Hook and Peter Pan), but they will take up a big chunk of your time. If you want to try to see WOC, I would strongly recommend a dining package, and you still will have to go get a spot about an hour in advance. For Fantasmic, if you don't mind spending the money, I would recommend booking the desert package. Unlike WDW, there is no "arena" for Fantasmic at DL, so you will see people staking out spots hours in advance. The desert package gives you actual seats in a seating area. You do have to line up to pick your seats and if you really want front row, you need to line up early, but it is still better than having to stake out a spot in my opinion.
Have a fun 2 days! We thought DL/CA would be a one time trip, but we fell in love with it, so we've been alternating between DL and WDW! You can do a ton at DL/CA in 2 days because everything is so close together. I'm not sure about the spring break crowds in your time period though. We have never been in March.
Be sure and take a look at Hydroguy's guide to DLR for WDW vets. It has some great info:
http://www.disboards.com/showthread.php?t=1162599&highlight=hydroguy+guide
Also, don't miss the Indiana Jones Ride- it is probably my favorite ride out of any of the WDW and DL/CA parks put together. The Aladdin show at CA is terrific too if it is running the days you are there. It is well worth the 45 minutes. We actually cancelled a trip to WDW and moved it to DL a couple of years ago when they announced it was closing (they later changed the decision), just so DD could see it before it was gone.