I'll give it a shot - I say: a strong federal government, which
  the USA has, resists major changes by having checks and
  balances, being rather slow to move with changes that directly
  affect the lives of citizens in a major way.... overall.

  Global Warming activism has been on a train towards becoming a
  global awareness/change situation for a very long time now, at
  least since the early 1980s when I was a kid and first heard
  about it. There were (and are) contrasting theories but the
  momentum has grown to where the UN is involved, the Pope made
  announcements (or will) and large powers around the world are at
  least talking about it, and likely to introduce various
  legislation to do something, because there are millions of eyes
  watching.

  ISIS, to me, is a threat for the military to take care of, and
  they are. Are they a threat to the USA directly or to the world?
  I don't think so. They are a splinter, radical group that, like
  Al Qaeda before it, likes to take credit for things whether they
  did them or not. The major war they play is over the media and
  making threats. They enjoy it when we make them larger than life
  because Image is Power and we inflate it. It works for us as
  well, because having an Enemy helps keep military coffers full.

  All that being said:
  Global Warming is a real threat.
  ISIS really does kill people and do nasty things.

  Legislative power rests in the hands of the House and the
  Senate; President can advise, but that's not much power in that
  way.

  Yet, the President can declare war with or without Congressional
  approval, so whoever is President next will likely have a
  _direct_ affect on our decisions overseas.

  Also, a president has the power to appoint judges, but that
  requires someone to die or retire. In that way, presidential
  influence can last for decades after they're out of office.

  So in short:
  President MAY make a difference with regards to ISIS.
  President will not have _as much_ influence on Global Warming
  legislation, but will have some.

  Their biggest power is their face and their voice.

  People listen. People make changes based on what they say which
  have little to do with the layout of the government.

  So, as an inspirational force, the next President *does* make a
  difference, both to those who agree, and to those who vehemently
  disagree with whoever is next.

  I also could've said, "No, I don't know what is the greater
  threat." I don't believe either one will have a direct impact on
  the lives of most American citizens.