This is the one I bought recently. Though I didn't do my research thoroughly enough and got a bit disappointed it doesn't charge quickly. But always good to have a bunch of extras for friends and trips where I won't be able to charge my packs as often.

It looks like the deal on the battery you listed is for the next 4 days. So there's no losing out it seems, since the 12th is in that time frame.

I'm currently tossing up between the Powercore 20100 (link) and the Powercore 20000 (link) (same thing, but QuickCharge 3.0). Is there any reason I shouldn't get the 20000?

If you need ~20000 mAh, then:

https://www.amazon.com/dp/B00VLNT9KW/

https://www.amazon.com/dp/B01B645YYK/

Yes, the Aukey one is only QC2.0 but it still not the worst charging speeds. You'll also save >$10. The RAVPower one you linked is also just as good. Just choose based on price and features.