Which DC-DC converter for a uController
Since the battery voltage is always higher than the required voltage, you need a "buck" or voltage reducing regulator, assuming that you want to use a switching regulator.  However, unless maximum efficiency is required, it would be much less complicated and costly to use just a 3.3V LDO (Low Dropout Regulator).  These can only reduce voltage.

An LDO uses a series pass element such as a MOSFET or transistor to drop the right amount of voltage from the input to produce the required voltage at the output.  The only real down side is that the dropped voltage is dissipated as heat, with the total power of the voltage drop times the current.  Since you have a relatively small drop and probably relatively low current, an LDO would not waste that much power.