I use the Pro Mini M8 by Magic-Pro. Its 8,000 mAh. What I like is that its slim due to its using Lithium Polymer cells.
Do some research pros & cons between Lithium Ion & Lithium Polymer cells, especially behavior of the different cells and charge capacity versus physical size.
Cost me rm199, but love it. Has dual USB out (1A & 2.1A) and can charge both iPhone 5 & iPad at the same time.
Link for reference http://www.magic-pro.com/eng/product/productDetail/665