Disneyland Park has more rides than Magic Kingdom (including some that disappeared from or were massively changed at MK years ago, such as Mr. Toad's Wild Ride and the Tiki Room). The castle is smaller, though.
Disney's California Adventure has some of the rides that MGM has (such as Tower of Terror), plus some others.
As far as feel, Disneyland is of course much smaller than WDW. It's all on a small piece of property in the middle of a busy city. But there are advantages to that.
You can stay "off site" but actually be directly across the street from the entrance.
You can "park hop" between DLP and DCA by simply walking from one side of the entrance plaza to the other.
The advantage of WDW is that it's huge, with very diverse offerings. It's a true resort, with all that implies.
"Disneyland Resort" isn't what most people would think of as a resort. It's two theme parks surrounded by hotels.
But Disneyland has the original of many things, plus some unique things such as the Matterhorn and California Screamin'. And it has a much better Space Mountain (darker, smoother) and a Pirates of the Caribbean ride that's nearly twice as long as the one at MK.
So if you're into the rides, Disneyland is definitely worth a look. If you're into the history, then Disneyland is also a good choice because that's where the Disney theme parks all started. Checking out the differences between, for example, the DL and MK Haunted Mansions can be fun. And the Imagineers' main campus is located right nearby, so DL tends to benefit from that in some ways.
My choice would be to see DL if you haven't and if what it has to offer - history, a better version of a few rides, and some rides you haven't yet experienced - interests you.
If you mostly go to WDW for the resort aspects, though, then Disneyland probably won't be worth it.
And one more caveat: if going to Disneyland means you won't be able to go to WDW for a long time, I'd personally stick with WDW.