I too would recommend the boost regulator and the use of a single Li-po cell. Using a single cell makes the li-po charger requirement simpler as there is no worry about a two cell battery going 'out of balance' or requiring a special 'balancing' charger set-up. Just size the single cell mAH rating for the run time duration the project requires. In this day and age with inexpensive switching regulators no one should be using old fashion linear voltage regulators with battery powered projects.
Lefty