@Arctic_White did you decide yet? I missed this thread I guess because May 1st was our MY LR pick-up date so I was a bit preoccupied for community browsing.
My vote is for the 2021 LR, unless for some reason you think you need that extra second off the 0-100 time. Are you planning to race every sports car you see? If so, then I guess yes get the P. Aside from the extra go, the wheels are something that draws people to the P, but you're planning to remove them and downsize anyway. As I understand it, the P has a harsher ride (and trust me--not sure what the streets are like one province over, but around here the LR is rough enough as it is) which isn't just from the wheel diff but also suspension tweaks.
Here's another thought: assuming the original owner got that P in late September, why are they giving/have they given it up in about 9 months time? Are there issues with it? Are you taking on someone else's problem vehicle? I still vote for the brand new LR. It's plenty fast and can be made somewhere-in-between faster with the AB if you decide you need it. It's got the right size wheels, it's got the colour interior you wanted, it's got all the production upgrades and build quality improvements that have been made since September, and it's rolling off a truck with your name on it with [basically] 0 clicks on the odometer.
EDIT: forgot to say: assuming the two options really are similar in price, that means the MYP has taken somewhere around a $10-15k haircut (+/- some after taxes and so on). Are you really sure it's going to hold its value better for a few years?