Is it better to boost or buck?
For this application a buck or boost converter would be overkill. Your best option would likely be to have a 12v source and then take it down to 5V separately with a linear regulator. Cheap, few parts, most likely parts you already have, etc.

Dropping voltage with a linear regulator generates heat based on the current drawn through the regulator and the voltage dropped. This should not be an issue as the PIC will likely not draw much current.

However, a simple "cheat" way to drop the voltage enough that the regulator won't have a large voltage drop across it is to put one or more diodes in series before the regulator and use them to drop the voltage by ~.7v to 1.4v each depending on the diode. Dropping the voltage to 7v for a 5v regulator should be fine and allow enough headroom for the dropout of the regulator. Again, simple design and parts that you might readily find off the shelf or even salvage from old stuff.

Cheers