Should I charge my iPhone 6 with 5V/ 2.4A or 5V/ 1A?

The standard charger is 5 volts @ 1 amp (= 5 watts). Using a higher wattage such as the charger that comes with an iPad won't hurt the phone,

http://www.apple.com/shop/product/MD810LL/A/apple-5w-usb-power-adapter?fnode=97

As long as you have 5V and I suggest an Apple charger rather than copies ,(unless MFi ( Made for iPhone certified)) the amperage is not important

Apple shows all Apple iOS device chargers as compatible with all iPhones.

DeathReaper4411 wrote:

I'm not really sure what the difference is and which one is safe to use since looking around, I couldn't find much with 2.4A

The 6/6S can take advantage of up to 1.6 amps if the USB adapter can supply it. Using a higher capacity adapter will not hurt the phone, but won't help either. An iPad adapter is 2.2 amps, the iPhone "cube" is 1 amp. So an iPad adapter will charge an iPhone 6 or 6S series slightly faster. Not true for the 5 and earlier, which will only use 1 amp regardless of what is available.

Why my iPhone 6 is finish it charge in locked screen?

Ba Reen786 wrote:

Why my iPhone 6 is finish it charge in locked screen?

I don't understand your question.