I've been to WDW 7 times with 55 park days. I've been to DL several times now with over 20 park days (most have happened over the last 18 months). WDW is still my favorite place to go, but with a family of 5, it's easier and cheaper to drive 350 to DL. Below is some things I can speak to on what's better and worse.
What you lose by going to DL: All of Animal Kingdom - I miss this park with the safari and Everest and all the animals and great Broadway type shows. DL does not replicate any of this park with the exception of one of the Dinoland rides. World Showcase - I do wish DL had something like this. Irreplaceable. Rides and shows - Laugh Floor, Stich's Escape, New Fantasyland, Indy Stunt Show, LMA Car Show, Tower of Terror is a lot better at WDW, Figment's Ride. Complete Immersion - Without spending your kid's college money and staying at Grand Cali, you just don't get that Disney Bubble feeling at DL. The DL Hotel gets you close, but I still didn't feel in the bubble like I do when I stay at any of the WDW moderates or deluxes. Nightly Entertainment - Wishes, and Fantasmic! every night....Fireworks and and F! at DL happen only on the weekends during the offseason. And no Illuminations, which for me is the best. Table Service Options - While DL has one of the best TS (Blue Bayou sitting on the banks of PiratesOTC), you certainly don't have all the great places like Liberty Tree Tavern, Tony's, Crystal Palace, or anything from World Showcase.
What you gain by going to DL: Cars Land - This place really is amazing. While the Racers ride is a lot like Test Track, it isn't quite as fast. But it replaces it by far with the most immersive ride you'll ever go on. Period. Add in the other rides, eats, and overall architecture and it's an amazing place. Everything is withing walking distance - This really can't be overstated. Whether at one of the DL hotels or at a hotel on Harbor Blvd, being able to not have to wait for a bus/rush for that bus going to where you want to go is a piece of stress I don't miss at all. PLUS, if you want to parkhop, no bus is needed. Exit the gate, walk the distance of a football field, enter the gate, BOOM! It's that easy. And while you don't have the grand size of WDW, the small size of DL just makes everything so convenient. Rides and Shows - Pirates, Space Mt, and Big Thunder are all better versions than the WDW counterparts. Finding Nemo Subs, Matterhorn (if you have a good neck), Mickey & The Magical Map show, Aladdin Live, Bug's Land (very well themed and great for little ones), World of Color, a better Fantasmic! with far worse seating. Weather - for me, there's only a small window to go to WDW. It's too darn hot and rainy. In SoCal, kiss the humidity good bye. And you'll only find a handful of days where there's unbearable heat, unlike WDW where it's unbearable mid-May through mid-October.
Hope this gives you some good direction.