Those quoted voltages are nominal.  At full charge, the battery
will be 4.1V or 4.2V, depending on the way the phone terminates
the charge.  So these batteries are absolutely identical in
quoted capacity (real capacity might be a different matter).

Termination at 4.1V will increase the overall life of the
battery, but 4.2V will give the battery a little extra capacity
between charges.  Note that this limit is determined by the
phone, not the battery.

John