Series I versus Series EE

Where can I find comparisons on how Savings Bonds are doing against one another, for example, should I get Series EE or I bonds? And how long does it take for these bonds to mature or reach their face value, ie. a $100 dollar bond purchased for $50, how long till it is worth $100? A cashier at my bank told me they were now taking up to 21 years to reach face value. Is this correct? Tom's response My book has a complete analysis of the Series EE vs Series I question. Series I bonds are purchased at face value and don't have the value-doubling feature. Series EE bonds are purchased at one-half face value ($50 for a $100 bond) and are currently guaranteed to double in value in 20 years. This represents a guaranteed interest rate of 3.5% if you hold the bond for the full 20 years.