It won't matter which park you do on Saturday, they both are going to be extremely busy. The weekends are the busiest times for both parks. The time you arrive on Saturday most Fast Passes will probably be gone. I found it works out best to do some of the more popular attractions an hour before the park closing, then they are almost walk on or very minimal wait. This goes for both parks.
Going Monday morning you should be good if you're there right at park open. They do have some "early" entrance but it's something like 15 mins before the park opens, nothing special.
I hope this helps Disneyland and DisneySea are both fun parks and you'll really enjoy it!
TDR Explorer: Exploring Tokyo Disney Resort. Advice, Planning, Photos, and more. TDR Now: The first English speaking Podcast focusing on Tokyo Disney Resort
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