I've been to all three parks within the last couple years. I like all three but they are very different from DL and each other.
Magic Mountain is THE roller coaster park for So Cal! It has something like 20 roller coasters including some of the largest and fastest in the country! Disadvantages--- The only really practical way to get there is to rent a car for the day. It is about an hour from DL straight north on the 5. Because it has so many coasters it attracts a different crowd than DL and is not as clean and pretty, but is decent. Also food generally sucks so eat outside the park. It will be hot (they have misters in line) if the current weather holds. It is supposed to hit near 100 today.
Knotts is a lot closer, has some coasters, but not as many as MM. I like it there it has some great rides and shows (love Mystery Lodge) but large parts of the park are just shops and carnival type rides. Clean? Well, it's not DL, but it's ok. Best thing there I think is the Chicken Dinner Restaurant which is outside of the park. Love those biscuts.
Universal has grown a lot in the last few years. Still not that many rides and only one that could be called a coaster, and it's not that big of a thrill. Still it's a fun park with lots of shows and and some great rides. It is sort of between Knotts and MM are far as distance from DL. If you go, don't miss T3, Jurasic Park and Back to the Future. Again food is not their strong point, but there are some great places in Universal City Walk just outside the park.
All three are fun in their own way and could be done in one day. Check out their web sites for more info. Have fun deciding and be sure to post a trip report (especially about the cruise) when you get back. We are on the June 25th sailing.
Cheryl