I think those external battery packs probably are more practical as you could get some insane mAh rating one which would replace 2-3 spare batteries and not require powering off your phone to use it.

Like I said though, those solar ones I am dubious about as the one I had, despite being left in the window for weeks in direct optimal sunlight, never charged. Yet the charging led was always lit and the battery clearly worked as it charged fine from a mains adapter and also I later dismantled it and found the solar panels also worked fine.