Others have helped you with sage advice.
When I am traveling and reliant on technology I carry a spare everything. Hope for the best, plan for the worst. I like Anker's reliability and their good warranty but anything can fail either inside the product or I break it.
For this reason if I'm away and reliant on tech I'd rather carry say two Powercore 10000 than one Powercore 20000. In my personal situation I find I do not need as much as 20000mAh, more usually about 10000mAh so I might be with say a Powercore II 6700 + the Powercore+ Mini.
Good luck. But needing luck less often is luckier.