How much draw (current) does the iPhone have while being charged.

Both will work. By the way your ports are providing 5Watts from one port and about 10Watts from the other port. Your iPhone 6s will handle that with ease. NOTE that the iPhone will control the total electrical power it needs to charge the battery. It will not overcharge. The 5Watts port will charge slower than the 10Watts port that you have. Both will work fine.

Axel F.

It's my understanding that with USB-A, the maximum input current for something from an iPhone 6 to iPhone 8 Plus should be 2100 mA. Starting with the iPhone 8 there's difference when a USB-C power adapter is used with a Lightning to USB-C cable, but that shouldn't apply to the iPhone 6s.

You can find that out if you have a Mac. I know that my iPhone 7 maxes out at 2100 mA. This is from an Apple Support article. It's 2100 mA from the 500 mA standard operating current and 1600 mA supplemental operating current. In sleep mode a Mac will provide the full 2100 mA as "sleep current". It doesn't say which model though.

If your computer doesn't recognize your iPhone, iPad, or iPod - Apple Support

It should be safe to use either port, but obviously it's going to be current limited with 1A. My wife used to use her 6s Plus with the 5W "cube" power adapter that came in the box, but when I gave her my 10W power adapter that came in the box of an iPad Mini 2, she remarked at how much faster it seemed to be charging.