To charge a phone with 3000mAh battery the powerbank will drain by almost 3800mAh because there is almost 20-25% powerloss when charging a battery, so plan your purchase accordingly.
I got a romoss sence 4(10kmAH) with an lcd display, it charges 1700mAh battery 3 times with juice left for more.
Now there are Li-Polymer batteries available in the market, they weigh considerably less then the Li-ion batteries and have a higher claimed service life but are a little expensive.