"Lucifer's Hammer" is about what happens after a huge natural disaster and how mankind struggles to stay alive and reorganize.
As I understand it "Footfall", having picked it up a few times without reading it, but answering entirely from memory is concerned with an alien invasion.
WARNING & DISCLAIMER:
SELECT AND USE OUTDOORS AND SURVIVAL EQUIPMENT, SUPPLIES AND TECHNIQUES AT YOUR OWN RISK. Information posted
on this forum is not reviewed for accuracy and may not be reliable, use at your own risk. Please
review the full WARNING & DISCLAIMER about information on this
site.