Definately missing something. If you only know one and then go to the other you get both the feeling of something familiar and something different.
WDW is bigger and has much more to offer: Magic Kingdom, Epcot, MGM, Animal Kingdom, Water Parks, Downtown Disney, and many many resorts.
Disneyland is smaller and more intimate. Disneyland itself (comparable to the Magic Kingdom at WDW), California Adventure (different than anything at WDW but to me part Epcot and part MGM), and Downtown Disney. 3 unique hotlels but no water parks.
My first experience was WDW and it is still my favorite. That doesn't stop me from enjoying the Disneyland resort. To me what makes it most special is that Disneyland is Walt's park. He walked tht park hosted parties in that park, and even had an apartment in that park. That alone makes it special.
I encourage you to go visit Disneyland and I encourage your brothers family to give WDW a try.