Creating a 5 volt (ish) power pack from 18650 batteries
2 cells in series provide twice the energy of a single cell, and current is halved so capacity will be a bit higher. Buck converters are also usually more efficient than boost converters, so in practice it will probably be more than double. 2 cells in parallel will probably provide a bit less than 2 in series.

One possible downside of cells in series is charging. If they are charged in situ then the charger must put out up to 8.4V, and a balancing circuit is required. This means you won't be able to use those cheap TP4056 modules, and the charger may need its own power supply rather than working off USB. 

Balancing is not required for cells in parallel. However 'BMS' functions to prevent over-charge and over-discharge are always required. For long cycle life the discharge voltage should not be allowed to go below 3.0V.