I can't tell you how many threads I've read about WDW vs DLR. There are interesting points to both sides, and I can understand them both.
WDW-Definitely more of a destination vacation, i.e. more resorts, more parks, more to do in general. I LOVE WDW, and we go there every single year without fail.
That said, I have been lucky enough to go to DLR every year of my childhood, and nearly every year in my adulthood (save a couple years, childbirth, surgeries, etc.
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DLR is different than WDW in so many ways, but one of the biggest, IMHO, and most important is the true sense of being the "original."
I mean, walking through the castle (SO tiny compared to WDW) and to know that Walt himself walked through it, dreamed it and created it, is a special "magic" I simply can't explain. The original "rustic" look of Frontierland and adventurland, and the DL railroad all hold such a special lock on my heart, it simply cannot compare to anything WDW has to offer. Don't get me started on the candy shop people, it is likely my favorite place in the entire world (not just Disney) to be. Seriously, I stand and watch (and eat) so long in front of the window where they make candy, I sometimes think they're going to tell me to take a hike
The monorail actually going into the park is a HUGE piece of magic for me too. I thought when they took the monorail out of the DLH I was gonna' die (STILL think that was a horrible idea, but I digress). The submarine ride, now that it's back as Nemo, was the first experience I ever had with Disney. And watching the tops glide through the water, listening to the peaceful music and watching the monorail glide in front of the Matterhorn, what can I say...that is my magic.
If I absolutely HAD to give up one for the other, I would give up WDW. Although we go there more, and stay longer, it simply doesn't hold the same magic for me and my family.
Also to add, we never stay @ DLR for less than 8 nights. We easily spend 3 days @ DL and 2+ days @ DCA. We also head to the beach for a half day, but other than that, stay on property. There are so many great resort activities, pools, DTD and so forth, that I can say for myself and my family, it is definitely not a 3-4 day trip. Not if you want to really take in the magic and enjoy it..just my .02.