Dumb question - are you really going to UO for two full weeks in December and three more in August??
I only ask because I LOVE Universal and even I think both of those are way too long for a single visit.
Or are you mixing a few days of Universal into your trip?
If you are sincerely going for multiple weeks at a time, then you do not “need” UXP. It will be busy that time in December. August will still be busy, but fluctuates depending on school return dates. (Typically the last couple of week are slower until the Thursday before Labor Day.
Universal is definitely a “locals” park. It gets massively busier on the weekends.
The best advice I can give you on getting a Universal AP is this…
You can ALWAYS upgrade your AP. You can not downgrade it.
Get the least expensive pass that doesn’t have blackout dates for your trip(s). If you think you are missing out on benefits then spend more.