In both 2017 and 2018,
Disneyland Park reached capacity and closed to new visitors on December 27 and 28, but not on the dates that you'll be there. Of course, that does
not mean it will happen the same way this year, but it's interesting that it was the exact same dates both years. But who knows? It's certainly possible the parks will reach capacity on your dates, and it's impossible to predict.
However, there is good news. First, Disneyland did apparently allow people who had previously entered (earlier in the day) to re-enter in 2018 (though not in 2017) even after they closed the gates to new visitors. And California Adventure
didn't reach capacity either of those days. And I'm not aware of it ever reaching capacity and closing the turnstiles (someone correct me if I'm wrong). Again, there are no guarantees, but it seems less likely that you would be effectively locked out of
both parks if you had been there earlier in the day. But this might actually be a reason to get Park Hopper -- just in case you did get locked out of Disneyland, you could (probably) get into DCA.
For me, the answer to this is hand's down MaxPass, and that would be the case
any time of the year. But it's doubly so when it's busy. Anything that can help you get your FastPasses when it's that crowded is going to be a huge help. It's true that you could still get FastPasses the old fashioned way (from the kiosks), but it just makes it that much harder to maximize your use of them (being able to get a new FastPass as
soon as you're able, instead of waiting until you're done with the attraction or navigate to the next kiosk).