If you are only going for one day, I would suggest a 1-Day Park Hopper. Yes, DCA closes earlier than DL, and for the price you'll pay for admission, at least get a hopper so you can get your money's worth. Many visitors rush the DL gates once DCA has closed.
There's a lot of fun things at DCA, but if I had to spend a full day at either park, it would be
Disneyland. DCA is a lot smaller, and it's easy to see all the important stuff (if not the entire park) in less than a day, maybe even several hours. I've been there several times during different seasons and it never seems as crowded as DL, also meaning the lines aren't as long - fast passes help too.
DCA is a good place for smaller kids, if you are traveling with small kids. DL is too of course, but if you're going with adults only, things may get a little yawny for them at DCA, which is where a park hopper would come in handy.
DCA also focuses more on Pixar, Disney channel stuff, and California / Surfin' fun in the sun themes, where as Disneyland has a more original and nostalgic feel. There's some newer mainstream things there too, but at least High School Musical 2 isn't constantly in your face.