It doesn't make any difference but I would go with the higher mAh. It will give more charge life to your iPhone. In other words, you will get more charges to your iPhone from the external battery before the external battery runs down. I think the mAh of the iPhone internal battery is 1200 mAh so if you get the 1000 mAh it will not fully charge the iPhone battery. Here is another one I just learned about. It is 5000 mAh. The 1900 mAh is a good enough external battery for the iPhone though.