I'm making plans to be in LA during the first week of March and "MUST" visit a National Park. My goal is to see all the US National Parks. I'm pressed for time and can't do both Catalina and the Channel Islands....but can't decide which to do. Right now I'm leaning towards Catalina, staying one night on the island and going to the Channel Island visitor center the next day on my way to Santa Barbara. I'd appreciate any ideas or thoughts. Thanks in advance.

Virtually all hotels on Catalina Island require a 2 night stay. You might find a 1 night stay in March since its still low season. Do you have a boat? If not, Catalina is not on the way to Santa Barbara. It is southwest of LA off the Orange County coast.

Just a heads up, but Catalina Island isn't part of the National Park System.

If you'll be in LA, you might want to consider driving down to Joshua Tree NP. Maybe you'd get to see some wildflowers, although I don't think the bloom is supposed to be very good this year.
http://www.desertusa.com/wildflo/ca_jtnp.html

You should def. head out to the Channel Islands, I think you'll enjoy yourself much more than just going to the visitor center.

Ian
http://www.inationalpark.com

As much as I like Catalina Island, I think for your purposes visit one of the Channel Islands per inationalpark. And that is a great time of year to visit Joshua Tree NP.