As others have said, it really depends on how you enjoy your days. At WDW do you go to parks commando style, park hop daily, rope drops, late nights, go back to resort to relax or swim, take time to dine table service nightly or what?
While DLR has just two parks, it packs a ton into those two parks and with Carsland opening soon its even more.
Disneyland itself has quite a bit more than Magic Kingdom.
Park hopping is extremely easy at DLR. The two parks, 3 resorts and Downtown Disney are all in quick walking distance from each other. No buses, boats or monorails needed (monorail does go from Downtown Disney to Tomorrowland inside Disneyland but can walk instead).
For us 3 days is minimum, 4 days preferred but then we've been there dozens of times and don't need to see the smaller attractions. If you like to take it easy and relaxed, time out for TS dining etc. then you will want to stay longer. If staying a full week we visit Knotts, beach or Universal for a day. Sometimes we stay in San Diego for 2 or 3 nights to visit Seaworld, Zoo and beach there then move to Disneyland area for rest of week.