Here is my background: grew up in California and have been to DL many, many times. Have taken my daughter to DL three times and have taken her twice to WDW. Those two trips are my only experience with WDW- the most recent was this past March.
DLR is compact- both parks, DTD, the Disney resorts and many off site hotels are all with in walking distance. This has advantages and disadvantages. Because there are only two parks, you dont have the stress of trying to figure out which park to go to an a particular day to avoid crowds, whether to do EMH or not, or where to make your ADRS months in advance so that you are not trying to get from one part of the world to another to eat. DL is always more crowded than DCA, it opens earlier, and it has the only early entry opportunity, which you should use if you have the option. When DL gets too crowded for you, then you walk over to DCA. If you like the resort feel of WDW and spend a good portion of your vacation at your resort lounging by the pool, then I would expect that you would not be as happy with the DLR experience, especially if you are not staying on site (GC and DLH both have nice pools- it is hit or miss at the surrounding hotels- some are, literally, in the middle of the parking lot). If you love eating at the restaurants at WDW- you will not be as happy at DLR. While there are a few good restaurants and character meals, there is not the variety found at WDW. It seems to me that eating is a BIG part of the WDW experience - not so much at DLR. You don't have anything like the World Showcase choices, Sci Fi drive in, the 50s primetime, or the place at Wilderness Lodge where you might end up with scads of bottles of ketchup on your table.
I agree with the above posters that if you want to go onto Disney property and not leave for a week, then you should go to WDW. There is much to do and see in the SO. Cal area, however, you can easily spend a week to ten days there and barely touch the surface. You can also expand your trip to other parts of California and see some sights that everyone should see at some point in his/her life, such as San Francisco, Yosemite Valley, and the California coast. I see that you are from the UK, so am assuming perhaps incorrectly, that you dont make many trips to California. This might be an opportunity.
If your favorite park at WDW is EPCOT, then you probably wont be very happy at DLR. We have Soaring at DCA and the Nemo ride at DL has some similarities to the one at Living Seas, There is a small Innoventions pavillion in Tomorrowland -but it doesn't compare very well. If your favorite park is Animal Kingdom because of the animals, you wont be happy at DLR. We have a couple rides that are similar to AK rides: Grizzly River Run is, IMO, vastly superior to Kali River Rapids; Indiana Jones is same ride system as DINOSAUR but a completely different themed ride. But there are no animals at DLR (except for the horse pulling trolleys on mainstreet). You would have to head down to San Diego to get a comparable animal experience.
If, however, you go to Disney to spend time in the parks and love the attractions, I think you should come experience DLR. With one notable exception (Tower of Terror) whenever the same attraction exists in both DLR and WDW, the DL version is better than or equal to the WDW version(again- IMO!). A couple like the HM are just different enough that you can end up liking both equally well. Like the other posters said, DL's Fantasyland is far better than WDW's. On the downside, it was not designed to handle the volume of people that it now attracts and can get extremely congested with strollers.
If I was forced to choose between DLR and WDW resorts and pick one resort to go to for the rest of my life- I would choose DLR. Maybe there is nostalgia in that decision as I went to that park as a child, but I think it has more to do with the feel of the park itself. I am charmed by DL in a way that MK does not come near. I would miss Epcot and AK, but not as much as I would miss DL. My daughter, who is now15, would choose WDW "because it has more options." So let's hope my family never has to make this choice.

Good luck with your trip planning!