Powerbank for Ipad pro 12.9?

As Rudegar states, it doesn't really matter but know that the iPad Pro has a 10,307mAh battery. You'd be best off getting an external battery of at least 12,000mAh with 2.4 amps. This should charge the 12.9 a little more than 1/2 a charge...power is always lost in conversion, heat and cable resistance...those need to be taken into consideration when calculating. Efficiency is on average around 60%. To figure this:

External battery's mAh, divided by device battery mAh, minus 60% (this is using an iPhone calculator...others may require you to subtract 40% rather than 60% to get the correct outcome)

20,000mAh external battery:

20,000 divided by 10,307 = 1.940 subtract 60% = 1.164

You should get ~1.16 charges out of a 20,000mAh external battery

12,000mAh external battery:

12,000 divided by 10,307 = 1.164 subtract 60% = .69

You should get a little more than 1/2 charge out of a 12,000mAh external battery

You'd be better off with at least a 20,000mAh or the maximum (allowable battery size for use on airlines I believe) of 26,800mAh. The iPad Pro 12.9 is a beast as far as it's battery goes. You'll want the most power available to you to charge it effectively.

ios devices don't see any difference between an AC charger and a powerbank apart from when the power bank is empty it's like the flip of the switch

which one is best depend on size, looks and power storage wishes