If you only have the ability to get a one park pass (though it would be highly encouraged to get a park hopper pass), I would decide if you’d prefer the quintessential Disney experience (thus choosing Disneyland) or a more adult experience (alcohol, bigger rides, etc. and thus choosing California Adventure). Are you only doing Disney for a day or are you planning on multiple days? *Source: I’m an Orange County native and my husband used to work the ticket booths at Disney*
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In that case, I’d definitely get a multi-day park hopper. You’d be able to either spend one day in each park, or sample both parks, all on one ticket. They’re both different experiences, so I’d encourage you to visit both since you’ll be there for more than a day.
I personally have gone to Disneyland and we did both parks in one day and by the end we were exhausted and I missed out on rides I really wanted to go on. If you can split into 2 days and be careful when booking as Disneyland is still closed.
As of right now Disneyland is not open and has said yesterday they don’t have plans to reopen in the near future. They are in the most restrictive tier in California.
If you’re planning on going late 2021 or even into 2022 I think you’ll be ok. Disneyland is classic, but California Adventure has a couple more high speed fun rides. Which ever you choose will be great! And the park hopper may come back by the time you go so you could do both.