I almost always get a park hopper. I have only not one time and we went to DL. DCA is really cool because it's only in CA. Although most of the attractions can be found in other parks in WDW, Cars Land is only here at DCA, at least for now.
My advice would be to get a park hopper and maximize your use of fast pass and the single rider lines when available. Also do things that you know for sure you don't want to miss first.
Try and get there when the park opens (buy your ticket the day before or online so that you won't have to wait in line for that too).
Tomorrow
Disneyland is open from 8 am - 6 pm (then Mickey's Not So Scary Halloween Party starts so the park is closed unless you buy a separate ticket for that) and DCA is open from 8am-8pm. So if you have to leave by 8 pm you aren't missing that much of park time anyways.
If you are going to get a park hopper I would start out in DCA and get a fastpass for Radiator Springs Racers (your fast pass will probably be for later in the day unless you are one of the first people in DCA that day because the FP go super fast for that attraction), after you get your FP head right over to Toy Story Mania, there is no FP for this ride and the line gets very long. If there's time before your FP you can also do California Screamin' or Little Mermaid (there is usually no wait for Little Mermaid). Then hopefully it'll be time for your fast pass and you can do RSR and then head over to Disneyland and pick up a FP for one of the more popular attractions (Space Mountain, Thunder Mountain, Indian Jones, Star Tours) all of those lines get really long.
Keep in mind there will probably be a lot more people in the park around 3-4 pm because the Halloween party guests can get in at this time, but they will only be in DL so theoretically DCA should be less crowded at this time. Try and get all you want to do in Disneyland done before 4-5 pm, utilize the fastpasses and try to get a new one right after your return time starts for your current one, and then head back over to DCA for anything you missed the first time (TOT).
Most importantly have fun!
