You're going to hear a lot of heartfelt, emotionally charged reasons that one park is better than another in the upcoming posts, so I'll try to remain objective.
"Disneyland is the original!" The park's aficionados will scream that as a reason that it's better than WDW's MK until they die. To be fair, Disneyland does boast many more attractions than the MK (WDW had multiple parks long before DCA was built); and the park's unique style of small, intimate, highly detailed theming still makes it the most charming park the Company owns. However, depending on when you go, be prepared to encounter huge crowds: that small, intimate setting also creates traffic headaches. If you've never been to DL but you've been to WDW several times, enjoy the unique attractions it offers. Indiana Jones, Alice in Wonderland, Roger Rabbit, and Pinnochio all inhabit attractions there that we don't have.
Other comparisons abound. WDW's MK is definitely more spacious and beautiful, with wider expanses of flowers and trees; DL is cramped. Most of the MK's attractions (Splash, BTMRR) are improved versions of their DL counterparts; but DL's PotC is much better than the smaller version down here, and they still have the original, much better Tiki Room, as well as Mr. Toad's Wild Ride (but the MK's version was better). DL also features more sit-down restaurants and has been completely refurbished for its 50th anniversary; but before the management change, DL was literally falling apart. DL also features areas not found in the MK, such as New Orleans Square, Critter Country, and a vastly superior ToonTown (unlike ours, their version was never intended to be temporary). Because they don't encounter Floridian sun and rain, DL's Fantasyland facades are much more beautiful than the MK's; yet when WDW was constructed, the MK's Fantasyland was better. And our Tomorrowland beats the pants off of theirs, regardless of whether or not they have a new Space Mountain. We still have the TTA, the CoP, and live bands. They have empty buildings, a half-built Innoventions, and some stupid Club Buzz thing (they are, however, getting their subs back).
In case you can't tell by the way I keep going back and forth, DL and the MK are simply two different parks. With its small structure and established vegetation, DL is more charming; and some of their rides (especially PotC) are better than ours; but the MK is large enough to feel like you're really somewhere else (DL, in contrast, is pretty much off the side of the road), and we don't have to cram everything into one park because WDW has four.
I like both of the Kingdoms for different reasons. WDW's MK is still my favorite, but that's personal opinion. If you've never been to DL, go! Especially with the 50th celebration, you won't regret it. DL is every bit as magical as the MK.
Just be prepared for how tiny their castle is. It's cute, but it's tiny.
