I've got the Juice Pack Plus and I have been able to charge my iPhone 4S from 29% to 98% with one light left. I love the Juice Pack Plus! The White one looks awesome too. It's a shame because I have the Black and Pink but I want the White one now too. Terrible, I know.
During a black out, my husband's Juice Pack Plus had one light left and I had 33% left. I was able to use his remaining power to charge my iPhone 4S to 61%. Not bad.
I've never owned the Juice Pack Air, but I bet its just as good of a product. It might not charge your phone to 100%, but no matter which you get, its great to have on the go. Mophie makes great products.
If your on the fence because of money, I found a coupon code worth 15% (Coupon Code - power15) see below.
When I purchased my Juice Pack Plus, I had a code. I always look for discounts and coupon codes. NEVER pay retail if you can help it! LOL!