The most important number is the voltage .. it must be 5v
The next is the amount of amps that the charger is capable of supplying ... this must not be less than 1 amp (1000mA) in the case of a phone and must be greater than 2 amps (2000mA) for an ipad.
So in theory (cough) the greater the amps a charger can provide .. then the better quality / higher rated the components are, which should mean that it will have a longer life.
So the answer as to which one is better = if they are both 5 volt then only the 2100mA one is suitable
EDIT..
Just for clarification, as a lot of peeps get confused..
The voltage from the charger is very very very important.. (if in any doubt then always use a genuine Apple product)
If the voltage is correct then the phone will control how many amps (or parts of an amp) that it wants from the charger, so as long as the charger is capable of providing that amount of current (expressed in amps) then all is well.